A keycap forming method includes the following steps. Firstly, a stabilizer bar is fixed on a first male mold. Then, a keycap is formed through a second male mold and a female mold according to an injection molding process. Then, the second male mold and the female mold are separated from each other, so that the keycap is fixed on the female mold. Then, the first male mold and the female mold are stacked on each other, so that the stabilizer bar and the keycap are combined together. Then, the first male mold and the female mold are separated from each other. Consequently, the stabilizer bar is detached from the first male mold. By using the keycap forming method, it is not necessary to manually assemble the stabilizer bar with the keycap.