The invention discloses a subway shield gas tunnel construction method, which comprises the following steps: 1, dividing a gas tunnel to be constructed into one or more harmful gas pressure sections from end to end; 2, respectively constructing the harmful gas pressure sections in turn according to the construction work stages of the gas tunnel to be constructed, wherein construction processes ofone harmful gas pressure section are as follows: calculating limit gas emission and correspondingly judging the level of the gas tunnel, and modifying the explosion protection performance of a shieldmachine, the ventilation quantity of a ventilation system and a gas monitoring control system; setting the driving speed of the shield machine; performing shield driving construction; and performing duct piece assembly and lining construction; and 3, completing constructing all the harmful gas pressure sections of the gas tunnel to be constructed until realizing the run-through of the gas tunnel to be constructed. The method has the advantages of skillful and reasonable design, convenient implementation, small construction difficulty and good construction effect, and can effectively solve theproblem of safe construction for running through a shield tunnel of a gas reservoir, namely a subway shield gas tunnel.