The invention relates to a precise adding device for gas drilling. The device is communicated with a high-pressure vertical pipe (14) of a drilling circulatory system through a parallel vertical pipe (12), a pressure valve (3) and a pressure valve (13) are arranged at connection ends of the parallel vertical pipe (12) and the high-pressure vertical pipe (14) respectively, the parallel vertical pipe (12) is communicated with a tank (8) through an adding barrel, the bottom of the tank (8) is fixedly connected with a motor (9) through a support frame (10), and an output end of the motor (9) is connected with a screw (11) arranged in the adding barrel. The sealed tank is used for storing solid particle materials; the motor is used for controlling the rotating speed of the screw rod, so that the adding speed is controlled precisely; the valves at two ends of the parallel vertical pipe can control addition and pressure relief of the tank. The precise adding device is applicable to precise addition of treating agents to shafts during gas drilling.