With the development of short distance communication network, multimode fiber (MMF) has become a ideal medium to realize high-speed transmission of information with large-capacity. But the severe model dispersion of the MMF restrict its transmission ability. To improve the transmission ability of the MMF, a MMF communication system based on adaptive modulation (AM) orthogonal frequency division multiplexing (OFDM) is designed. An adaptive bit loading algorithm which is fit for this communication system is proposed. Simulation is done to validate the feasibility of the algorithm. Based on the given system, the effect of adaptive modulation to the transmission performance of the system is emphatically analyzed. And the bit error rate (BER) curves of different bit rates, different transmission performance with and without adaptive modulation are compared. The result shows that adaptive modulation can improve the transmission performance of the system effectively, and overcome the effect of deep fading nulls very well.