A method based on thermo-optical modulation of ultrasonic surface waves generated by laser for detecting crack of aircraft engine blade is presented. Crack closure on the blade due to laser-heating is considered in this method, thus two series of ultrasonic pulses including closure situation and original situation are both detected by interferometer when scanning the line source focused from NdYAG laser. The signals under heating by modulation signed with Sheat are compared with the cooling ones signed with Scool. The difference value between Sheat and Scool exhibits the effect of the crack closure because of the optical-thermal modulation. The results demonstrate that the crack on aircraft engine blade can be detected with reliability and high sensitivity.