Abstract: The bistable state occurrence has been investigated theoretically for squeeze film damper(SFD) supported flexible rotor system.The bistable state calculation method of the system is presented in detail.The bearing parameter B ,stiffness ratio f and unbalance U are chosen to analyze the bistable state performance,and a domain of U(B,f ) is obtained.The results show that the bistable state phenomenon can be avoided as long as the value of U beyond the above mentioned domain.This makes it convenient to optimize the SFD design parameters.