Leakage from no.30 drainage gallery at left abutment of Xiaolangdi hydropower station was studied using tracing methods of environmental isotopes, chemical composition, etc. The result was made by the means of analyzing the data of environmental isotopes, direction of groundwater, temperature field, etc, which was that the leakage water of drainage holes in the north of no.30 drainage gallery might come from the reservoir water passing through the fracture zone of the fault F28 and then inrush into permeable T1 3-1 stratum. The result was confirmed by the interconnection experiments. The research result about the leakage from no.30 drainage gallery would play a great role in designing curtain of left abutment.