Abstract: As an important component of the innate immune response, the complement system has biological significance that goes far beyond simple, non-specific, defense mechanisms. In the tumor microenvironment, the complement system plays a dual role in the occurrence and development of tumors, affecting the outcome of the immune response. Through the regulation of immunity and direct killing of tumor cells, the complement system promotes immune surveillance and inhibits tumor progression; however, an over-activated complement system can affect the immunity of the host, which is an important link among inflammation, immunosuppression, and tumor progression. Targeting to complement contributes to overcoming immunosuppression and leads to an antitumor immune response. This article reviews the recent advances in research on the relationship between the complement system and antitumor immune response with the aim of providing choices for antitumor immunotherapy.