In order to maximize the transmission rate with the certain rated power. This paper analyzes the channel characteristics and parameters of 2-100MHz broadband power line communication under the conditions of low-voltage power line communication channel. Using existed power line communication channel model, the iterative block constraint allocation algorithm can dynamic allocate subcarrier of power line communication, compared with the traditional water flooding algorithm, improved algorithm can better constraints for subcarrier allocation according to the power line communication channel power and get better channel capacity.