To solve the problem of noise in signal, an event detection method of optical time domain reflectometer（OTDR） based on improved wavelet transform is proposed. Firstly, by improving the wavelet de-noising algorithm, a new threshold selection method and threshold judgment function are obtained. Then, the event location and the beginning end location of the fiber are measured by combining the wavelet mode maximum method and the slope legal bit, and the event attenuation loss is calculated while the event is separated and the misjudgment is eliminated. The simulation and experimental results show that the proposed method has high positioning accuracy, and the calculated position deviation of reflected event and non-reflected event is less than 5 m and 15 m, respectively. The results of event detection and analysis of the measured fiber are consistent with the connection position and the total length of the fiber.