The current three-band terahertz filter have a problem of large difference in passband bandwidth. In order to improve the bandwidth uniformity of terahertz filter, a frequency-selective surface based terahertz filter is designed, which consists of three nested "cross" ring metal patches, "cross" metal patches and polyimide substrate. When the terahertz wave is vertically incident on the filter surface, three filter passbands are formed in the range of 0.1~0.862THz, and the peak transmission coefficient of each passband is about 0.9, which has good passband performance, and the transmission coefficient of transmission zero between the passbands is below 0.07, and the out-of-band suppression is good. The maximum bandpass ratio of 3dB bandwidth is about 1.29, and the maximum bandpass ratio of 10dB bandwidth is about 1.13, which has good passband uniformity. The rectangular coefficients of the three working passbands are greater than 1.7, the passband edges are steep, and the frequency selectivity is good. The filter has the characteristics of high transmission coefficient, small bandwidth difference, simple structure and easy processing. It has broad application prospects in the field of terahertz multi-channel communication.