In OFDM system,a bit and power allocation method based on bit-preloading is proposed in order to achieve a result with minimum Bit Rate Error(BER) under the constraint of total transmission power and data rate within a shorter calculation time.It is proved that the bit-loading result of Minimum Total Virtual Power(MTVP) is the same as that of minimizing BER.Then an iteration method aiming at the MTVP is proposed,in which the preload of bit according to BER threshold is used to decrease the iteration times.The simulation shows that the proposed bit and power allocation method based on preload approaches the optimal solution as well as computes efficiently.