A room-temperature-operation tunable continuous-wave terahertz (THz) radiation and detection system, which consists of two external cavity semiconductor lasers and an interdigited finger bow-tie antenna on low temperature GaAs substrate, is established based on photomixing and coherent detection technology. The generation and detection of continuous-wave THz wave are realized, and the spectral purity of system is better than 1 MHz. The signal-to-noise ratio (SNR) of the system is better than 50 dB with frequency below 0.70 THz, and the maximum SNR of 64 dB is achieved at 0.50 THz. In addition, the relation between the electric field intensity of the radiated terahertz wave and the biased electrical field for the photoconductive antenna is experimentally given, and the impact of multi-longitudinal mode operation of the external cavity semiconductor laser on continuous-wave THz radiation is discussed.