Influences of system parameters of the piston-crankshaft system in an internal combustion engine on the main journal vibration are investigated with the proposed systematic method considering oil film forces. In doing so, oil film forces of the lubricated piston pack and main journal were solved and then incorporated into the dynamic equations for the components in the system, through which the system parameter influences on the main journal vibration are simulated, and further verified. The numerical results show that there exists an optimal rotational speed of the internal combustion engine to keep the main journal stable. Moreover, the varied ratio of the connecting-rod length to piston diameter, unbalance mass of crankshaft, and damping coefficient of the main journal can change the main journal’s stability, and its vibration amplitude.