The current dual-band frequency selective surface (FSS) structure has limited passband frequency selection performance. In order to improve the performance of the terahertz band passband filter device, a terahertz dual-passband filter based on FSS is designed. The device is mainly composed of three layers of concentric metal copper rings in a regular quadrilateral, with a dielectric substrate loaded in the middle. The thickness of the metal layer and the substrate are both 0.04λ, where λ is the wavelength of the low frequency band, and the unit size is 0.15λ. According to the equivalent circuit method (ECM), the reason for the dual-frequency passband of the structure is analyzed, and the structural optimization of the design is further discussed. The geometric parameters of the original structure are equivalently adjusted by changing the parameters of the equivalent circuit components. The simulation results are basically consistent with the theoretical calculations. The optimized device can achieve passband filtering at two center frequencies of 2.74 THz and 8.17 THz. The band range at lower frequencies with insertion loss less than -1 dB is 2.24~3.25 THz, where the maximum insertion loss is only -0.05 dB. At higher frequencies, the band range is 7.63~8.71 THz, where the maximum insertion loss is just -0.12 dB. The steepness of the filter characteristic curve is good, and the relative bandwidth exceeds 10%. The structure has small size, good frequency selectivity, and high angular stability. It can be used for broadband filtering in the terahertz frequency band, and has important application value for future terahertz communications.