Magnetic semiconductor materials have potential applications in spintronic devices. In this work, some nano-device structures based on the magnetic semiconductor NiBr ₂monolayer (NiBr ₂-ML) are designed, their spin-resolved transport and photoelectric properties are studied by using density functional theory combined with non-equilibrium Green’s function method. The results show that both the NiBr ₂-ML PN-junction diodes and sub-3 nanometer PIN-junction field-effect transistors (FETs) exhibit the significant rectification and spin filtering effects in either the armchair or the zigzag direction. The gates can obviously tune the electron transmission of the PIN-junction FETs. The current is significantly suppressed with the increase of gate voltage. In addition, NiBr ₂-ML has a strong response to the blue and green light, thus its phototransistor can generate a strong photocurrent under the irradiation of blue and green light. The research results in this paper reveal the multifunctional characteristics of NiBr ₂-ML, which provides an important reference for the application of nickel-based dihalides in semiconductor spintronic devices and optoelectronic devices.