Abstract: The nonlinear dynamics model of the crankshaft system including 6 linkages,6 pistons and one propeller is established with the consideration of alternate moment of inertia of the linkages and pistons,and the coupling effect between torsional and axial vibration.The governing equations represent a strong nonlinear and non autonomous system.By numeric simulation,the response of the system to the initial displacement and velocity,the variable moment of inertia,the pressure applied on the piston,torque and axial force applied on the propeller is analyzed individually.It is found that the variable moment of inertia and coupling effect between torsional and axial vibration is the essential reason for nonlinear vibration of the system.Different exciting factors can not only cause different frequency components,but also different amplitude for the same frequency component.Under the special conditions in the paper,the dynamic characteristics of the system is not influenced distinctively by the actuating torque and axial force applied on the propeller.The sensitivity of the response in different direction to the same exciting factor is clearly different.