Abstract: In the process of penetrating the reservoir by underbalanced drilling, with the increase of the penetrated reservoir thickness, the volume of gas kicking into the wellbore increases, which causes the drastic change of wellhead void fraction, leading to the bottom-hole pressure out of control. The changing of void fractions in different well depths are calculated during underbalanced drilling, and the influence of different air intake flow and wellhead back pressure on wellbore void fractions is analyzed. In consideration of wellhead pressure adding value on bottom hole pressure, the computing method of wellhead pressure of constant air intake flow is obtained for penetrating the reservoir by underbalanced drilling. The results show that gas rapidly expands when it reaches the surface, and the wellhead back pressure have a enormous effect on the void fractions at wellhead while have a little influence on the void fraction under 500 meters. Keeping the air intake flow constant, with the increase of the penetrated reservoir thickness, the wellhead pressure increases. The smaller the air intake flow, the bigger the wellhead pressure needs to load.