Machine degradation may cause product quality degradation in production. Therefore， a joint optimization model of lot-streaming scheduling and preventive maintenance considering the relationship between product quality and machine degradation was established. The machines were assumed to have two states of "controlled" and "out of control". The stochastic dependence between upstream machines and downstream machines was established using the unqualified products produced by "out of control" machines. Based on the above-mentioned descriptions， a mathematical model with an objective function for minimizing the makespan and the total expected cost was derived. The improved differential evolution algorithm was designed to solve the model. Finally， the convergence of the algorithm and the effectiveness of the model were verified by numerical results and the comparative experiments respectively.