Abstract: Due to the environmental complexity of some construction operations and the uncertainty of personnel operations, vicious incidents caused by construction machinery crossing the nearby live safety distance still occur, which has a bad impact on the stable and reliable operation of the power system. Therefore, an early warning system for near electrical safety distance in power system is studied. First, the basic working principle of the safety distance early warning system based on critical electric field threshold is introduced. Then, by establishing a three-dimensional finite element model of the power system on-line operation, and taking typical 35-500 kV AC transmission line as examples, the electric field threshold change characteristics under different transmission line levels and different operating positions are simulated and analyzed, so as to provide a basis for the nearby electric warning. Finally, the electric field strength and safe distance are measured on the spot using the developed system. The actual measurement results well verify the early warning system, which show that when the system generates an alarm under a critical electric field, the actual minimum safety distance measured is 0.7-1.6 meters larger than the standard value, and the safety margin can reach 20%, which is better than similar technologies. The research results can provide technical support for the safe and stable operation of the power system.