The invention provides a generator prime mover system damping online detecting method. The method includes the steps of S1, calculating generator dissipated energy flow and unit damping dissipated energy flow flowing from a power grid into a generator; S2, calculating excitation dissipated energy flow flowing from the generator into an excitation system; S3, calculating prime mover dissipated energy flow flowing from the generator into a prime mover system according to the difference between the dissipated energy flow flowing from the power grid into the generator and the excitation dissipated energy flow; S4, respectively performing linear fitting on the prime mover dissipated energy flow and the unit damping dissipated energy flow, and calculating the slope ratio of the prime mover dissipated energy flow to the unit damping dissipated energy flow to obtain the damping torque coefficient of the prime mover system. The generator prime mover system damping online detecting method has the advantages that the dissipated energy flow flowing from the power grid into the generator and flowing from the generator into the excitation system is calculated to indirectly calculate the prime mover dissipated energy flow flowing into the prime mover system, and the damping torque coefficient of the generator prime mover system is calculated online.