In this paper, numerical simulation technique was used to simulate the potential distribution when the X70 steel was influenced by stray current of different strength. The simulation results suggested that without cathodic protection, X70 steel was obviously anodic polarized in the current-out zone, with the potentials of -636mV, -559mV and -500mV respectively when the stray current was 15mA, 50mA and 100mA. The anodic polarization degree increased with the increasing of stray current density. The potential of the current-out zone was about -1000mV, -870mV and -780mV respectively when cathodic protection was adopted, and X70 steel was fully protected as the stray current corrosion controlled. The accuracy of simulation was proved by comparison simulation results with practical measurement results, which showed that the numerical simulation technique was an efficient, quick and convenient means for stray current corrosion study.