The invention relates to a nondestructive testing system for defects of a composite insulator based on the reflecting property of a microwave band. The system comprises a femtosecond laser source, a wave guide device, a switch port, an oscillograph and a data analysis module, wherein the femtosecond laser source is used for generating detection signals; the wave guide device is used for connecting the femtosecond laser source with the switch port; the switch port comprises a partial wave mirror surface with bidirectional permeability to microwave band signals and is used for separating incident signals and reflected signals; and the data analysis module is used for carrying out time-domain analysis on the incident signals and the reflected signals, simulating and calculating basic conditions of a composite insulator at a tested point so as to realize defect determination. By utilizing a new composite insulator defect detection manner, internal defects of the composite insulator can be effectively detected.