The current situation of generator loss of excitation protection and the usual scheme of loss of excitation protection based on impedance principle are introduced. By analyzing the defects of the usual scheme， the feasibility of improving the performance of impedance principle based protection is analyzed and discussed. The improved scheme of generator loss of excitation protection based on impedance principle is detailed focusing on the operation characteristics of the impedance circles， low excitation current element and accelerating element. At last， the dynamic model test of a real digital protection relay is briefly introduced.