Under gas-gun loading, the change of light transmittance of liquid benzene under multiple shock compression was observed online by light transmission method. The results show that the light transmittance of benzene remains good under the first impact loading; After the second impact, when the pressure reaches 2.27GPa and the temperature is 543K, the light transmittance begins to decline; With the increase of pressure and temperature, the light transmittance decreases more obviously. It can be seen from the phase diagram that the liquid benzene enters the phase region of benzene II after the second impact. At this time, the crystalline phase transition of benzene sample occurs, which leads to the change of light transmittance.