Switch mode power supplies(SMPS) are widely used in programmable logic controllers, household appliances, etc. due to their high integration and light weight, and becomeing the most popular power supply products. The voltage sag will affect the output voltage of the SMPS and the working state of its power supply equipment. The paper study the influence of SMPS on voltage sag by measureing and analysising modal. Firstly, A large amount of dates obtained by the test were used to draw the voltage tolerance curve(VTC) of SMPS; and then, divideing the magnitude-duration flat into five different modals combined with VTC and referenced to control variables; finaly, Qualitative analysis of voltage waveform at the output of SMPS under different modals. Revealing response mechanism between the voltage sag at the input terminal, the output voltage waveform and the operating characteristics .