In order to improve the reliability of overcurrent protection of coal mine, for 6 kV outlet switch of substation of all levels, this paper gives an improved setting calculation method of overcurrent protection. It analyses the shortcomings of traditional method and introduces setting calculation process of the new method. The new method uses the line ampacity to set the timing overcurrent and overload protection, and introduces the judging method of setting instantaneous overcurrent protection or not. Taking the timing overcurrent protection as an example, it introduces the processing method when sensitivity does not meet the requirment. Finally, the local power supply system of a mine of Henan Da You Energy is taken as an example to calculate and demonstrate, and then make analyses. The results show that the new method is effective and practical, and it can guarantee the overcurrent protection device action reliably, so as to improve the reliability of overcurrent protection.