The invention relates to a coal underground gasification process system. The coal underground gasification process system comprises a gasification agent injection drill hole, coal gas outlet drill holes, a main gasification channel, branch gasification channels, a drill hole supporting tube, a gasification agent injection tube, a combustion agent injection tube, a sealing device and an ignition point shifter. The gasification agent injection drill hole is connected with the coal gas outlet drill holes through the main gasification channel and the branch gasification channels, and the combustion agent injection tube is sleeved with the gasification agent injection tube and is connected through the sealing device. A gasification process includes the steps of carrying out ignition, judging whether water vapor is fed into, feeding into water vapor, judging whether an ignition point is moved forward, starting the ignition point shifter and continuing to conduct combustion. The coal underground gasification process system has the advantages that the transverse gasification area and longitudinal gasification area can be enlarged, coal reserves for gasification are increased, autoignition and controllable back gas injection are achieved in a well-free coal underground gasification process, meanwhile the problem of short circuit of gasification agents in the gasification channels is solved, the probability of back mixing of product gas and the gasification agents is lowered, and safety operation of the gasification process is guaranteed.