The loss-of-excitation protection of synchronous generators is one of important protections in the power system relay protection. The excitation protection is a complex problem because the excitation faults are related to the large-disturbance stability of generators. The operational situation of the current excitation protection hasn't been ideal and so needs further study on. The shortage of three excitation protection criterions applied currently is analyzed， which based on small-disturbance stability analysis principle without considering the changes of dynamic power-angle character of the generator， or without considering a rather large common area between measuring impedance circle after complete loss-of-excitation and measuring impedance circle after a disturbance at normal excitation operation. So the maloperation or misoperation is difficultly avoided. Based on the simulation analysis of dynamic varying process of synchronous generators after losing excitation， an improved scheme based on direct power-angle measurement is put forward， including setting conditions and calculation method of loss-of-excitation protection of synchronous generators. Simulation results show that the presented protection scheme can judge excitation fault reliably and rapidly， and at the same time， its operation is stable with flexible and easy setting mode.