Two-dimensional (2D) ferromagnetic semiconductors have been recognized as the cornerstone for next generation electric devices, but the development is highly limited by the weak ferromagnetic coupling and low Curie temperature (Tc). Here, we reported a general mechanism which can significantly enhance the ferromagnetic coupling in 2D semiconductors without introducing carriers. On the basis of a double-orbital model, we revealed that the superexchange-driven ferromagnetism is closely related to the virtual exchange gap, and lowering this gap by isovalent alloying can significantly enhance the ferromagnetic (FM) coupling. On the basis of the experimentally available two-dimensional CrI3 and CrGeTe3, the FM coupling in two semiconducting alloy compounds CrWI6 and CrWGe2Te6 monolayers are calculated to be enhanced by 3-5 times without introducing any carriers. Furthermore, a room-temperature ferromagnetic semiconductor is achieved under a small in-plane strain (4%). Thus, our findings not only deepen the understanding of FM semiconductors but also open a new door for realistic spintronics.