Frequency selective surface (FSS) structure is theoretically analyzed by spectral domain method. A composite slot FSS bandpass filter with the center frequency of 0.338 THz is designed in the terahertz communication window. The filter is achieved by using triple element instead of the center disk of the circle slot FSS bandpass filter. In order to optimize the structure and then improve the filter properties,the relationship between structural parameters and filter properties is discussed. Results show that the comprehensive performance of the composite slot filter is better than the circle slot one. The 3 dB bandwidth reaches 69.5 GHz,the return loss is less than -13.46 dB,the maximum insertion loss in-band is less than 0.45 dB and the ripple is as low as 0.1 dB. The passband is flat and the sideband is abrupt. At the same time,the filter has stable filtering properties at different polarization waves and the same polarized wave at different incident angles within the scope of 30°. So it can be effectively used in the field of terahertz communication.