The invention discloses a testing device for gas desorption characteristics under an external liquid invasion condition and an application method of the testing device. A coal sample is loaded by using a coal sample tank; a constant temperature of the coal sample tank is kept by using a constant-temperature water bath box; an evacuating device communicates with the coal sample tank and is used for evacuating the coal sample tank; a methane injection device communicates with the coal sample tank and is used for injecting the coal sample tank into a methane gas; a water injection device communicates with the coal sample tank and is used for injecting water into the coal sample tank; a shaking device is connected with the coal sample tank and is used for driving the coal sample tank to shake to roll the coal sample; a desorption measurement device communicates with the coal sample tank and is used for measuring the gas desorption amount of the coal sample. The primary pore structure of the coal sample is not destroyed to really reflect the desorption characteristics of the gas of a mine site under the uniform coal wetting condition.