Recently, it was observed that cracking on the surface of concrete bridge pillars is more significant in Chinese high-elevation regions, where the climatic character is low air pressure. Many regions in the world are located in low air pressure climate conditions; however, the influence of air pressure on concrete has rarely been studied before. In this paper, mechanical properties and shrinkage of concrete exposed to different air pressures and relative humidities for 1 year were studied. Effects of air pressure on developments of mechanical properties and shrinkage properties were analysed. A modified environmental coefficient for predicting shrinkage of concrete was proposed. Under lower air pressure, lower mechanical strength and greater shrinkage strain were observed, especially after 28 d. The value of the ratio of shrinkage strains was almost constant during the aging of C30 and C50 concretes. The ratio was larger when air pressure was lower. The modified environmental coefficient, taking account of air pressure, was used to predict concrete shrinkage. This coefficient can be used in existing models in place of the original relative humidity coefficient.