There existed strong mode coupling when light traveling in the polymer optical fiber (POF). Due to the mode coupling , the transmission bandwidth of POF was enhanced. The mode coupling of POF was studied based on the power-flow equation. The mode coupling coefficient of POF was measured experimentally, the measured coupling coefficient was 7.61×10-4 rad2/m. The mode coupling process in POF was simulated numerically using measured coupling coefficient; a coupling length of 20 m was obtained. The real bandwidth of POF was revealed from the coupling length. The transmission bandwidth of POF was about 130 Mbit/s over 150 m. To testify the result, a 125 Mbit/s·150 m local area network (LAN) POF communication experiment was carried out, the transmission waveform and communication eye diagram were analyzed. The experiment results showed that the transmission bandwidth of POF was enhanced by mode coupling, the transmission distance of POF could reach 150 m at data rate 125 Mbit/s.