Considering the calculation of channel capacity of the step- index polymer optical fiber (SI- POF) communication system within the uncoded discrete multitone modulation (DMT), the intensity modulated direct detected (IM/DD) transmission scheme is mathematically modeled, and a novel uniform power loading technique is used to evaluate and calculate the maximum achievable date rate. With the help of Gaussian and first-order low pass filter channel model, the theoretical expressions for the optimal bandwidth and realistic capacity of SI-POF system are derived based on signal to noise ratio (SNR) gap. The numerical calculations with different equivalent SNRs and occupied bandwidths are compared and the real experimental transmissions with several fiber lengths are also implemented. The results demonstrate that the channel capacity is positively distributed to the equivalent SNR, yet inverse with the fiber lengths. Adopting the optimum bandwidth can effectively reduce the transmission rate loss. By the analysis, the accurate prediction of capacity is beneficial to the POF system designing.