Abstract: During gas drilling, the increase or decrease of annular diameter, partly balling up and section changing have great effect on annular flow field and cutting transportation .Through multiphase flow analysis and numerical simulation at key point of cutting transportation it was found that cutting was easy to hold up and pile up at the transition point of drilling collars and drilling pipe, the transition point of casing and open hole section, the increase and decrease diameter section and the balling up point, as they formed reflow area by the boundary layer separation. These areas may lead to cutting transportation difficult. While gas flew through cross section of drill collar and drill pipe, gas pressure and speed decreased and cutting transportation capacity is minimum, while the annular cuttings density is maximum. The minimum gas rate requirement for cutting transportation should be based on the minimum capacity at the key points.