Abstract: According to the Kurtzs theory and the relation between the transmittance of the polymer dispersed liquid crystal (PDLC) and the applied voltage, for the signal overflow during the testing process，we proposed using PDLC film to modulate the incident fundamental light intensity to control the signal of the second harmonic generation (SHG). The powder samples of potassium iodate, cesium vanadate and rubidium vanadate were tested and analyzed by this method respectively. It can be derived that KIO3 are phase-matchable, CsV2O5 and Rb3V5O14 are not phase-matchable. The signals of CsV2O5 and Rb3V5O14 is 8.13, 1.78 times that of potassium dideuterium phosphate(DKDP). The results show that this method is accurate, and easy to operate. More importantly, when applied to testing SHG coefficient quantitatively, this method can effectively improve measurement accuracy. It demonstrated that this method is feasible to test the non-linear characteristics of crystals.