Muon detector (MD) is water Cherenkov detector in Large High Altitude Air Shower Observatory (LHAASO) experiment. The liner is applied to seal 44 tons of ultra-pure water, which is the major detection media of MD. The liner must protect the water from contamination and inhibit bacteriological activities. The MD conducts a long duration mission (20 years), so any small leak would have severe consequences. The purpose of this paper is to introduce leak test for liners of MDs. A new method of $SF_{6}$ tracer gas leak test is proposed by using a $SF_{6}$ quantitative determination leak detector with high concentration sensitivity (0.01ppm). This method has quick response time and high sensitivity to detect small leak. Since 2018, 300 MDs have been installed, and no leakage has been found till now.