The embodiment of the invention provides an in-situ adsorption testing device for rock core gas and a working method of the in-situ adsorption testing device. The device comprises a constant-temperature tank for providing a required testing temperature, wherein a sample tank, a first pressure transmitter and a pressure loader are arranged in the constant-temperature tank, a gas adding device and a stress loading device are arranged outside the constant-temperature tank, and the sample tank is used for containing a to-be-tested rock core sample; the gas adding device is used for injecting testing gas into the sample tank until the pressure in the sample tank reaches a preset testing pressure value; the first pressure transmitter is used for acquiring the pressure and temperature data of the sample tank; the stress loading device is used for supplying pressure to the stress loading device; the pressure loader is used for converting the pressure supplied by the stress loading device into confining pressure and axial pressure which are supplied to the rock core sample in the sample tank. According to the embodiment, the confining pressure and the axial pressure are increased when the temperature and the gas pressure are considered, the test conditions are accordant with in-situ adsorption states of coal rock and shale, so that the accuracy of an adsorption test measuring result is increased.