According to the circuit of power supply system for grinding motor of double power rail grinding train, the requirement of selecting material for isolating transformer core was put forward. The relationship between core loss and magnetic density was analyzed by measuring the loss values of 30Q130 silicon steel sheet with magnetic density of 1.2 T, 1.3 T and 1.4 T at 120 Hz frequency. Based on core loss curves of 30Q130 silicon steel sheet, 23JGH100 silicon steel sheet and B20R070 silicon steel sheet, the surface temperature rise of iron core when three kinds of silicon steel sheet were used is analyzed and calculated. By comparing the values of magnetic density, loss and temperature rise of three kinds of core materials, the core materials and magnetic density of isolating transformer were determined.