Abstract: Low atmospheric pressure in the high altitude areas of Western China presents a new challenge to studies of the hydraulics of high speed flow in high dam construction. The trajectory distance of high speed jet nappes is a key technical parameter for safety design of plunge pools and bank slopes. This paper examines the influence of atmospheric pressure on the nappe trajectory distance using CFD analysis and develops an analytical formula for calculation of the distance under the condition of low atmospheric pressure in high altitude areas. The results show that lower ambient pressure leads to a farther nappe trajectory and the trajectory distance increases roughly by 1.5% when the ambient pressure drops by 10 kPa. Thus, particular attention should be paid to this ambient pressure factor when a relative short plunge pool must be adopted in valley areas to avoid safety risks of plunge pool and bank slope owing to the error in estimation of the nappe trajectories.