The invention provides a gas power device for a mechanical drilling rig. The gas power device comprises a power management system, a gas power machine, a gas generator set and a distribution room, wherein the gas generator set is connected with the distribution room of a drilling crew through a cable, and the power management system is connected to the gas power machine through a control wire harness; the output end of the gas power machine is connected with a torque converter or a coupler through a cross cardan shaft; the torque converter or the coupler is connected to a compound chain box through the cross cardan shaft, and the compound chain box is used for transmitting power to drilling crew equipment. The gas power device solves the problems of poor power responsibility and poor reliability of the conventional gas power device for the mechanical drilling rig. By adopting an electronic control gas engine is adopted, the gas power device ensures the responsibility of a power system. The original diesel power input end is connected with a set of gas compound power machine, so that diesel power replacement is realized. The gas generator set is adopted for replacement, so that the dynamic monitoring automation of power equipment is realized. Fault hidden dangers are reduced, and the working efficiency is improved.