Control over magnetic properties by optical stimulation is not only interesting from the physics point of view, but also important for practical applications such as magneto-optical devices. Here, based on a simple tight-binding (TB) model, we propose a general theory of light-induced magnetic phase transition (MPT) in antiferromagnets. Considering the fact that the bandgap of the antiferromagnetic (AFM) phase is usually larger than that of the ferromagnetic (FM) one for a given system, we suggest that light-induced electronic excitation prefers to stabilize the FM state over the AFM one, and will induce an MPT from AFM phase to FM phase once a critical photocarrier concentration (αc) is reached. This theory has been confirmed by performing first-principles calculations on a series of 2D van der Waals (vdW) antiferromagnets. Interestingly, a linear relationship between αc and the intrinsic material parameters is obtained, in agreement with our TB model analysis. Our general theory paves a new way to manipulate 2D magnetism with high speed and superior resolution.