The invention provides a method for detecting a seeping water source of an underground workshop of a pumped storage power station based on an isotope tracer technique. The method comprises the following steps: according to the overall structures of the underground workshop and an upper reservoir, collecting water samples from a measuring weir of the underground workshop, in-reservoir water, seepage around a dam of the upper reservoir, underground water, a gentle slope and the surrounding of the reservoir; analyzing radon isotopes in the water samples, at the same time, relatively analyzing theconcentration of hydrogen and oxygen isotopes, chloride ions, sulfate ions and nitrate ions in the water samples, wherein detection results of the concentration of the chloride ions, the sulfate ionsand the nitrate ions are up to the time point of half a year; analyzing all the contents in order to more comprehensively evaluate the seeping water source. The principle of fractional distillation of water isotopes is adopted for investigating a water supply source and distinguishing the composition of underground water from different sources in seeping water, which has a critical function in accurately understanding a seepage field of the dam and determining whether a seepage channel around the dame exists.