Abstract: Macrophages are important components of the innate immune system. Two major subtypes, M1 and M2, play a key role in the induction and regulation of specific immune responses, and in the development and recovery of diseases. Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are a class of severe respiratory diseases. In ALI, M1 highly activates and releases proinflammatory cytokines, which produce acute lung injury damage such as pulmonary edema and increase vascular permeability while fighting against pathogen invasion. M2 can be further subdivided into M2a, M2b and M2c subtypes according to inducing factors and their respective functions. M2a plays a role in promoting injury repair. M2b and M2c mainly function in immune regulation and suppression. M2c can also promote tissue repair. M1 and M2 can be transformed into each other. In the course of ALI, the balance between M1 and M2 is important to remove pathogens and promote tissue repair. The imbalance between M1 and M2 will lead to ALI deterioration, and even progress to ARDS. The present paper reviews the characteristics of different subtypes of macrophages and their relationship with ALI, the polarization and mutual transformation of M1 and M2, and the factors that affect the balance of different subtypes at different stages.