In order to clarify the cause of gas drilling downhole explosion during and the explosion limit of downhole mixed gas, according to the current gas drilling operation specifications and field experience, it is concluded that the annulus is not smooth or closed, the heat generated by the drilling tool and the rock friction, the sparks generated by the drill bit breaking the rock are the reasons for the downhole explosion. Combining the theoretical knowledge of combustion and explosion theory, a calculation model for the downhole explosion limit is established with thermal spontaneous combustion, hot plate ignition, hot ball ignition and spark ignition as the forms of ignition energy. Taking the con-struction of Well A as an example, the model can dynamically calculate the explosion limits of different well sections according to the changes in drilling parameters, which provides a basis for timely adjustment of the drilling plan and prevention and control of downhole combustion and explosion accidents.