For the on-line optimization of complex，large-scale chemical process systems，because optimization time may be too long or optimal solution cannot be obtained，the method of subsystem coordinating optimization can be an approach to solving the above problem.But interrelated variables of coordinating optimization may have different directions for a better solution.A coordinating optimization based on interrelated variables alternation is proposed.The subsystems are optimized based on alternation，and multi-problems of every subsystem optimization are solved by optimizing interrelated variables without changing independent variables or optimizing independent variables without changing interrelated variables.The alternation of subsystems is an iterative process until the condition of ending is satisfied.The proposed method is used in the optimization of simulated FCCU(fluid catalytic cracking unit).The solution is compared with the whole optimal solution，which shows the effectiveness of the proposed method.At last，the solution of on-line optimization with the proposed method shows its feasibility for chemical process on-line optimization.