In order to maximize the low power-line channel capacity under the constraints of bandwidth, a method of water-filling algorithm based on the specific QoS strategy is proposed. The multipath propagation model is used to derive the closed-expression of the maximum channel capacity under the condition of power limit. The analytic results show that the number of branches, the length of branches and different terminal loads have effects on the PLC channel capacity. Simulation results show that the mainly factors that affect the channel capacity of indoor low power-line are branch number and load impendence. In order to evaluate the performance of the proposed algorithm, a simulation environment based on OFDM is designed. The simulation results show that at the same SNR, the performance of proposed algorithm is superior to the existed water-filling algorithms. When SNR=40dB, the maximum capacity of the proposed algorithm is 1.37 times more than that of the classical water-filling algorithm, thus meeting the demand of high speed index in low power-line carrier communication system.