In the power transmission system, the main function of composite insulators is to achieve electrical insulation and mechanical fixation, that is, to prevent electric current from returning to the ground and support wires in the transmission line. It will be degraded in long-term operation under specified mechanical and electrical loads and various environmental conditions, and will also be subject to external shocks such as overvoltage and mechanical stress. The reliability of composite insulators exposed to the natural environment for a long time plays an important role in composite insulator manufacturers and line transmission. Aiming at the problem of aging and failure of composite insulators and considering the self-healing process of hydrophobicity, a dependent competitive failure model of natural degradation and multiple external shocks is established for composite insulators. The reliability and mean time to failure of composite insulators are studied, and the parameters in the model are estimated by the method of maximum likelihood estimation. Finally, the validity of the model is verified by numerical simulation.