Aiming at the problem of over-step tripping in Mugua Mine power supply system, the causes of over-step tripping were analyzed. Based on the analysis of the causes of over-step tripping, the solutions were given. Practice proved that the adoption of network intelligent relay protection technology against over-step trip and intelligent monitoring protector of mine high voltage distribution device with anti-over-step trip function could timely prevent over-step trip and ensure the stability and efficiency of the power supply system.