To quantify the influence of low pressure environment at high altitude on the cavitation characteristics of water flow, single gas nucleus critical pressure under different pressure conditions was theoretically calculated. The influence degree of single gas nucleus critical pressure on the nuclear incipient cavitation number was analyzed, and the sensitivity of flow cavitation number at different elevations was evaluated. Research results showed that, in low pressure environment, both gas nuclear incipient cavitation number and water flow cavitation number decreased, but the water flow cavitation number decreased much more intensively than gas nuclear incipient cavitation number did. On the premise that other conditions are the same, the cavitation erosion risk of high velocity flow is correspondingly high in high-altitude area. The research conclusions provide reference for the design against and evaluation of cavitation erosion of high speed flow in high altitude area.