Remaining Useful Lifetime (RUL) estimation is a significant part of health management. A RUL estimation method is proposed by combining the lifetime data with the degradation data based on diffusion process, and the RUL distribution is derived with the concept of the first passage time. In addition, the parameters are optimized offline and updated online. Firstly, maximum likelihood parameter estimation value based on the lifetime data is considered as the initial value of Bayesian updating. Secondly, the lifetime data and the degradation data are combined by Bayesian method to update parameters of the degradation model. Finally, the real time estimation of RUL is realized. The experimental results verify the effectiveness and superiority of the proposed method.