Abstract: Gas drilling has been widely used, because it can effectively protect low-pressure hydrocarbon reservoirs, solve the problems of severe well leakage and differential pressure sticking, and increase the penetration rate. Few literature reports related to the research on the mechanism of borehole deviation are made, so the mechanism is not quite clear year. Based on the stress status of the downhole rock during gas drilling, the controllable and uncontrollable factors influencing borehole deviation are analyzed, such as the temperature, the formation anisotropy, the borehole diameter enlargement, and the bottom hole assembly. The new rock-breaking mechanism and the formation anisotropy are considered to be the main factors influencing gas drilling. The borehole diameter enlargement of gas drilling exerts great effect on the bit lateral forces, therefore, it is another cause of borehole deviation. The control of borehole deviation is preliminarily discussed, which can be a good reference to the control and the design of bottom hole assembly. 