Southwest and northwest part of China has a large terrain height difference, relatively low air pressure. Hydraulic concrete is often exposed to low air pressure environment, resulting in a reduction of its durability. This paper investigated the influences of different air pressure environments on air content of hydraulic concrete and the performance of air-entraining agent solution. The results show that low air pressure significantly weakens the air-entraining capacity of air-entraining agent solution, and the rate of bubble decay is higher. Range and variance analysis show that in 101 and 80 kPa environments, the degree of influence of each factor on air content is in descending order: air-entraining agent dosage, water-cement ratio, and fly ash dosage. In 50 kPa environment, the influence of each factor on air content is in descending order: water-cement ratio, air-entraining agent dosage, and fly ash dosage. With the reduction of air pressure, the air content of hardened concrete decreases, the bubbles in the low air pressure environment are more likely to converge to form large bubbles, and the pore structure of hydraulic concrete is gradually deteriorated after hardening, which is specifically reflected in the serious deterioration of the pore structure of the specimen. The average pore diameter and average pore spacing coefficient of hardened concrete increase, and the air content of hardened concrete is significantly smaller than that of fresh concrete. The low air pressure environment retards the cement hydration process, resulting in a decrease in the stability of bubbles in the matrix, which in turn causes a decrease in air content of hardened concrete.