This paper describes the influence of pole-arc coefficient on electromagnetic vibration of automobile claw-pole alternator. No-load electromagnetic vibration of automobile alternator is studied by mechanical impedance method. The change of pole-arc coefficient to weaken the automobile alternator electromagnetic vibration force is proposed, so as to reduce the motor electromagnetic vibration. Different pole-arc coefficients of a 36 slots 12 poles automobile alternator are analyzed by the electromagnetic and structure finite element simulation. Theoretical analysis and simulation results show that automobile alternator electromagnetic vibration can be effectively reduced while choosing the suitable pole-arc coefficient.