During tunnel construction, the equipment and lighting can not work due to great voltage loss and low power factor caused by the fact the line of the power cable and lighting cable laid along the tunnel is too long. To improve this situation, power compensation is adopted to reduce voltage loss with the lowest cost based on the engineering practice. This method has a better effect, especially on the improvement of sever voltage loss upon long distance low voltage power supply. A discussion has also been made on the relationship between power factor and voltage loss, which can be use for reference.