The influence of extended light source on spectral reconstruction in the FTIR (fourier transform infrared spectrometer) based on step mirrors was discussed in the present paper. The relationship between coherent intensity and solid angle was calculated. It was found that the coherent length became shorter and the spectrum resolving power became lower as a result of solid angle produced by the extended light source. The spectrum resolving power of system could reach the value as the theoretic design if the solid angle of extended light source is smaller than 0.001. The radiance will reduce if the size of extended light source is shortened. Therefore, a suitable size of extended light source can be chosen, considering the requirement of SNR in the optical design.