One trench for agricultural irrigation next to the Hu-Ning Intercity Railway viaduct section were tested and analyzed in order to obtain the effect of ground vibration isolation when CRH trains run on. The result shows that with the decreasing train speed, the peak value of ground vibration velocity decreases and the main frequency of ground vibration heightens. Trench has lowpass filtering effect on ground surface vibration. The slower the train speed, the better the effect of isolation, but the phenomenon that ground surface vibration increases before trench is more obvious. When the length of trench is much longer than the length of train, the number of carriages has little effect on vibration isolation effect of the trench. When railways are built, consulting with nearby residents and department of agriculture, and the trench’s length, depth, breadth and the distance to railway should be designed reasonably. And then the trench in addition to meet need of irrigation, but also has very good effect on vibration isolation.