Due to the complexity of the interaction between boron and 3d transition metals, stable two-dimensional (2D) iron borides FeB_x compounds have attracted tremendous attention in recent years. Combining the evolutionary algorithm with first-principles calculations, we have systematically investigated the structural stabilities and electronic properties of 2D iron borides FeB_x (x = 2–10) alloys. It is found that the multilayer iron borides FeB_x (x = 4, 6, 8, 10) are wide-band-gap semiconductors, which are more stable than the corresponding monolayers. Furthermore, the electronic and optical properties of these semiconductors may be modulated by biaxial strains, indicating their potential application for advanced blue/UV light optoelectronic devices.