The concept of the lasing lifetime of working substance in the solid dye laser is promoted to liquid dye laser, and a twin laser beam testing method to measure the lasing lifetime is presented. Nd:YAG laser with the wavelength of 532 nm is used as the pumping source to simulate the pump condition in the dye laser. Low-power continuous-wave (CW) laser with the same wavelength is used to monitor the inactivation velocity of the dye molecules under the effect of pumping laser. Semiconductor refrigeration device, PID temperature control device and the water-bath are used to realize the thermostatically control of the dye solution. Lasing lifetime of Rhodamine 6G alcohol solution is tested while laser power density varies from 6.3×103 W/m2 to 2.2×104 W/m2. The experimental result indicates that the lasing lifetime of Rhodamine 6G is inversely proportional to the pumping laser power density. The proportional coefficient can be explained as the irradiation energy accumulated on unit molecule absorption cross section, when half of the dye molecules are dissociated. It is more convenient to use this coefficient to characterize the lasing lifetime of dye.