A kind of adaptive bit loading algorithm is proposed in this paper that is suitable for shortwave broadband orthogonal frequency division multiplexing (OFDM) system. This algorithm is based on Fischer's algorithm and improves it. A fixed transmit rate and a fixed total transmit power is assumed in this algorithm. As the assumption, the new algorithm can achieve the optimum performance of bit error rate (BER) on shortwave broadband channel. The simulated results show that the performance of the new algorithm is close to Chow's algorithm, but much simpler. Therefore the new algorithm is applicable to engineering realization.