A special designation is reported to measure the loss together of absorption and deflection in NLO crystals in the cavity. The method has characters of high sensitivity to about 10 －4 and fast response time up to 100 nanoseconds. We consider it extensible to be applied in the studies of nondestructive and quick destructive detection of NLO crystals related to the irradiation of high peek power laser. The results of KTP、KDP、KD*P crystals in our experiment and also the comparison to others is shown.