The invention provides an under-pressure well completion process method for a gas well, and belongs to the fields of whole-process underbalance and gas drilling well completion. The process method comprises a method for blowing a full-sealed flashboard chamber by using nitrogen before landing of an oil tube, a well completion tube string composition selection process under different relations of pipe column self-weight and jacking force, a process for safely replacing a Christmas tree after landing of the oil tube reaches a predetermined position and a well completion production process. The under-pressure well completion process method for the gas well reduces the construction risks of under-pressure well completion tube strings, avoids the damage of a replaced working fluid to a reservoir, and prolongs the production cycle of the gas well.