A coal rock permeability experiment system with adjustable outlet pressure comprises a methane gas tank, a confining pressure loading pump, an axial compression loading pump, a first three-way valve, a second three-way valve, a third three-way valve, a coal sample holder, a display, a pressure tracker and a computer, wherein the methane gas tank is connected with an inlet of the first three-way valve through a first gas pipe, one outlet of the first three-way valve is connected with a gas inlet of the coal sample holder through a second gas pipe, and a gas outlet of the coal sample holder is connected with a gas inlet of the pressure tracker through a third gas pipe; a first pressure sensor is in sealed connection at another outlet of the first three-way valve; a pressure reducing valve is arranged on the first gas pipe, and a mass flowmeter is arranged on the third gas pipe. A method is simple in process and convenient to operate, the pressure difference between a gas inlet and a gas outlet can be reduced when a higher-pressure permeability experiment is performed with the method, and a model error is reduced; according to the method, experiment parameters are acquired with an electronic technology, the precision of the acquired experiment parameters is high, and an observation error is reduced greatly.