Theoretical research and simulation analysis are combined to study the rationality of self-excitation judgment method for small hydrogenerator in local grid. Research reveals how the electro-mechanical process of generator’s load shedding influences the self-excitation and finds that the releasing power during rotor deceleration process may cause a extremely high transient self-excitation over voltage; and reveals how active load in local grid affects the self-excitation criterion of generator and finds the local active load will reduce the equivalent series capacitive reactance , making the system easier to meet the self-excitation conditions. At last, a simulation example is put forward as an evidence of the views of this paper. These conclusions are meaningful for accurate judgment of self-excitation in load grid containing small hydrogenerators.