Planar waveguide spectrometer is a compact spectrometer, which has an asymmetrical crossed Czerny-Turner optical structure within a planar waveguide structure. The multiply diffracted light of this spectrometer is large, so it needs to be carefully studied in order to find a suitable method to reduce it. The formed of the multiply diffracted light could be derived from the diffraction characteristic of grating. Then the stray light problem was identified by using the TracePro software. Then based on the difference of the spatial property and wavelength of stray light, different suppression methods were used. Filters and baffles were involved to suppress the multiply diffracted light, without changing the original optical structure. Finally, stray light coefficient was introduced to evaluate the effect using stray light suppression structure. Simulation and analysis results show that the multiply diffraction light of spectrometer has been well suppressed. The maximum of the stray light coefficient is reduced form 5.3% to 0.033%, and the average of it is dropped from 1.9% to 0.001 8%.