Aiming at the existing conditions that TVM430 monitoring system of electric locomotive on Qinghuangdao-Shenyang passenger railway line had only two speed-limited values of 160 km/h and 120 km/h, but there were three locomotive speed-limited values of 160 km/h, 140 km/h and 120 km/h in China, to satisfy practical requirements, a 140 km/h speed-limited device was provided as a supplement. The schemes and hardware/software structures of the speed-limited alarm monitor were introduced. Based on the studies, a prototype was developed and the test-running on train had good effects.