In order to overcome the existing disadvantages of offline laser peening detection methods, the online detection system based on laser plasma shock wave effects is developed. Laser is absorbed by the ablative layer on the surface of the work piece, and plasma of high temperature and pressure is formed under the tamping layer and propagated outward in the form of shock wave. The shock wave in the air is sampled, stored, digitally filtered and analyzed by the system. The system gets the sound pressure level factor to determine the effect of laser peening. The online laser peening detection system is designed and the implementation is performed. The system is very simple and convenient. By comparing the surface residual stress of work piece, the results show that the laser peening online detection system is sensitive and reliable.