Pumps-motor aggregate is the power unit of the ship ruder system, which requires high reliability and quietness. Misalignment of the pumps-motor aggregate induces abrasion of bearing and impact between stator and rotor, result in instability and characteristic line spectrum. Plum-shaped flexible couplings possess many advantages such as simple in structure, high compensatory ability and good damping capacity, which have been widely used in ship ruder system. Establishing parallel misalignment model and angular deviation misalignment model of plum-shaped flexible coupling, and analyzing vibrating characteristic of pumps-motor aggregate provide theoretical analysis and test data on misalignment status diagnose of pumps-motor aggregate.