The invention discloses a combined core for forming a plastic gear and an injection mold. The combined core comprises a first core body, a second core body, a third core body and a fourth core body which are coaxially arranged, wherein the second core body is positioned in the first core body, the third core body is positioned in the second core body, and the fourth core body is positioned in thethird core body. A coaxial positioning groove A is formed in the first core body and the second core body. A positioning block A for fixing the two core bodies is arranged in the positioning groove A.A flat opening A is formed in the side edge of the third core body. One end of the positioning block A is arranged in the flat opening A. In a narrow space, the four coaxial core bodies are positioned by using the positioning groove and the positioning block in the core, the processing of the core is facilitated, and the positioning accuracy of the core is ensured as well.