To minimize the electromagnetic interference of power line channel and to satisfy the requirement of power-line communication rate, this paper proposed a power and bit allocation algorithm based on Virtual Bit in power-line communication system. The algorithm first presents the concept of Virtual Bit and a rule as the optimal allocation criterion. Then it loads bits on sub-carriers based on the proposed rule, which achieves the optimal allocation target of the minimum power. The simulation results illustrate that the algorithm performance can achieve optimal and it is suitable for power spectrum limited high-speed power line communication systems.