Magnetic semiconductor materials have potential applications in the field of spintronic devices. In this paper, some nano-device structures based on the magnetic semiconductor NiBr2 monolayer (NiBr2-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 NiBr2-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 voltages. In addition, NiBr2-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 NiBr2-ML, which provides an important reference for the application of nickel-based dihalides in the field of semiconductor spintronic devices and optoelectronic devices.