Abstract: Based on performance tests and models on the under-excitation limiter（UEL）of main excitation regulator at home and abroad,damping torque formulas under constant reactive control have been deduced,including superimposed and comparative types of UEL.This paper analyses the mechanism of improving UEL dynamic stability by increasing an inertial filter link after active power measuring link.Besides,it is pointed out that damping torque is always a positive number under comparative type of UEL while under superimposed type of UEL,damping torque is related to parameters of UEL.Increasing proportional gain may enhance system damping and increasing integral gain may decrease system damping.Finally,by mixing physical tests with mathematical tests and eigenvalue analysis,it has been verified that adding an inertial filter link can greatly improve the suitability of UEL access to power system with default parameters.