Terahertz (THz) frequency upconversion detection technology is an important subject in the field of THz measurement. It offers several advantages, including the capability for room-temperature operation, fast response, high sensitivity, and high dynamic range.The nonlinear inorganic crystals suitable for THz frequency upconversion detection include MgO∶LiNbO₃ (LN), KTiOPO₄ (KTP), KTiOAsO₄ (KTA), and RbTiOPO₄ (RTP). The spectral range of the THz parametric source based on LN crystal can be 0.7~2.9 THz, while in the frequency range from 3.0 to 6.5 THz, KTP, KTA, and RTP crystals must be used in combination to achieve essential coverage. Currently, the characteristics of the THz frequency upconversion detection based on LN and KTP crystals have been reported. In this paper, the THz wave frequency upconversion detection characteristics based on RTP crystal are studied.The experimental setup of the THz parametric upconversion detection based on RTP crystal was mainly composed of three parts the pump source, the THz parametric generator, and the THz frequency upconversion detection part. The pumping source was a 1 064.1 nm Q-switched laser with a pulse width of 420 ps and a pulse repetition rate of 1 Hz. Two identical RTP crystals were used in the experiment. One crystal was used as the THz generator to generate THz waves, the other was used to realize frequency upconversion detection.The physical base of THz wave generation and frequency upconversion detection based on RTP crystal is Stimulated Polariton Scattering (SPS). The necessary condition for SPS is the existence of transverse A1 modes which are both infrared and Raman active in the crystal. The THz wave generation in RTP crystal was caused by the stimulated scattering of the polaritons associated with the most intensive transverse A1 mode of 271 cm^-1. But the existing of many much weaker transverse A1 modes of 211 cm^-1, 170 cm^-1, 161 cm^-1, 145 cm^-1, 107 cm^-1, 83 cm^-1, 51 cm^-1 made the Stokes and terahertz waves intermittently tunable. So the generated THz waves were mainly in four discrete wavelength regions, which were from 3.01 to 3.09 THz, from 3.53 to 4.17 THz, from 4.54 to 4.66 THz, and from 5.35 to 5.89 THz. The THz waves with center frequencies of 3.84 THz and 5.58 THz were selected for measurement in this study.In the SPS process, the pumping, Stokes, and terahertz waves must obey the energy conservation law and momentum conservation law. Because the refractive indexes of RTP crystal in the terahertz range are very large, only noncollinear phase matching can be realized. The angle between the pumping and Stokes beams (expressed as θ) and the angle between the pumping and THz beams (expressed as β) are dependent on the THz frequency. θ is very small, usually between a few tenths of a degree and a few degrees. However, β is rather large, distributed around 58°. The shape of the RTP crystal in the X-Y plane was designed as isosceles trapezoid with a base angle of 58°, a baseline length of 18.0 mm, and a waist length of 11.8 mm. The thickness in the Z direction was 5.0 mm. Two waist surfaces were both AR coated in the range of 1 060~1 100 nm. When the crystal was used as the THz generator, the pump light was vertically incident from one waist of the isosceles trapezoid, reflected by the bottom side, and left the crystal from the other waist. It was because of the design of the isosceles trapezoid with a base angle of 58° that the generated terahertz wave could be output almost vertically from the bottom side.The THz wave was focused and injected into the second RTP crystal through two off-axis parabolic mirrors (OAP1 and OAP2). The THz energy was attenuated by adding black polyethylene terephthalate (PET) plates between OAP1 and OAP2. A vertical slit was set in the THz optical path between the two parabolic mirrors, and the specific center frequency and bandwidth of the injected THz wave were selected by adjusting the transverse position and width of the slit, respectively. The upconverted signal spectral composition and intensity were detected by an optical spectrum analyzer and a highly sensitive photodiode, respectively.The upconverted signal pulse energy (expressed as the equivalent voltage displayed on the oscilloscope) showed a good linear relationship with the input THz pulse energy. The minimum detectable energy achieved at 3.84 THz was 2.29 fJ and the dynamic range was 52.7 dB. The minimum detectable energy obtained at 5.58 THz was 0.311 fJ and the dynamic range was 56.3 dB. The interaction length within the RTP crystal for the pump light, THz wave, and upconverted Stokes signal did not exceed 15 mm. The minimum detectable pulse peak power obtained from the RTP-based THz frequency upconversion detection system was comparable to that of the KTP-based system, while the interaction length for the pump light, THz wave, and upconverted Stokes signal in KTP crystal was larger than 90 mm. Regarding the frequency coverage of inorganic crystal frequency upconversion detection systems, the tuning range of a single crystal is discontinuous, the combination uses of RTP, KTP and KTA crystals can cover the spectral range from 3.0 to 6.5 THz.