Abstract: The single event transient (SET) effect of the 4N49 optocoupler was tested by pulsed laser experimental methodology. The characteristic of SET as amplitude and width was qualitatively investigated with the simulation of pulsed laser. A liner energy transfer (LET) threshold of 10 MeV•cm²•mg^-1 was measured when the bias voltage was 10 V. The saturated SET cross-section is about 1.2×10^-3cm². The SET effect of 4N49 optocoupler on the following digital gate circuit was validated. The circuit-level design technique for mitigation of SET is reasonable and effective, and the threshold LET of 7.89 MeV•cm²•mg^-12 can be improved to 22.19 MeV•cm²•mg^-1. The experiment study on the SET of 4N49 optocoupler provides an appropriate methodology to the SET test of the optics and their hardness assurance.