Detection for mode dynamics of fiber ring laser is a key problem for experiments and applications. On the condition of the different pumping intensity and long-time operation laser can appear complex mode dynamic phenomenon. Output of the intensity can be obtained easily based on the current experimental condition, but more exact mode dynamic characteristic is still difficult to be obtained. One type of mode hopping detection for fiber ring laser based on optic heterodyne is set up. Real-time mode hopping and frequency drift of fiber ring laser is detected. Time-frequency fall graph of mode characteristic and the mode intensity time series can be acquired. According to the experimental results, the output of fiber ring laser mode dynamics is detected and analysis for mode intensity can be obtained. This method plays an imperative role in laser control systems and frequency dynamics analysis.