Abstract: Cracks appeared on the reflector after the high and low temperature impact test of an optical module. The causes of reflector cracking were analyzed by means of macroscopic analysis, shear strength test, hardness test and linear expansion coefficient test. The results show that the improper composition of DG-4 adhesive resulted in great curing stress to the reflector, and the reflector was subjected to large thermal stress due to rapid rise and fall of temperature in the high and low temperature impact test. Combination of the two effects caused the reflector cracking.