Two-dimensional semiconductor materials with intrinsic magnetism have great application prospects in realizing spintronic devices with low power consumption, small size and high efficiency. Some two-dimensional materials with special lattice structures, such as kagome lattice crystals, are favored by researchers because of their novel properties in magnetism and electronic properties. Recently, a new two-dimensional magnetic semiconductor material Nb₃Cl₈ monolayer with kagome lattice structure was successfully prepared, which provides a new platform for exploring two-dimensional magnetic semiconductor devices with kagome structure. In this work, we study the electronic structure and magnetic anisotropy of Nb₃Cl₈ monolayer. We also further construct its p-n junction diode and study its spin transport properties by using density functional theory combined with non-equilibrium Green’s function method. The results show that the phonon spectrum of the Nb₃Cl₈ monolayer has no negative frequency, confirming its dynamic stability. The band gap of the spin-down state (1.157 eV) is significantly larger than that of the spin-up state (0.639 eV). The magnetic moment of the Nb₃Cl₈ monolayer is 0.997 μ_B, and its easy magnetization axis is in the plane and along the x-axis direction based on its energy of magnetic anisotropy. The Nb atoms make the main contribution to the magnetic anisotropy. When the strain is applied, the band gap of the spin-down states will decrease, while the band gap of the spin-up state monotonically decreases from the negative (compress) to positive (tensile) strain. As the strain variable goes from –6% to 6%, the contribution of Nb atoms to the total magnetic moment gradually increases. Moreover, strain causes the easy magnetization axis of the Nb₃Cl₈ monolayer to flip vertically from in-plane to out-plane. The designed p-n junction diode nanodevice based on Nb₃Cl₈ monolayer exhibits an obvious rectification effect. In addition, the current in the spin-up state is larger than that in the spin-down state, exhibiting a spin-polarized transport behavior. Moreover, a negative differential resistance (NDR) phenomenon is also observed, which could be used in the NDR devices. These results demonstrate that the Nb₃Cl₈ monolayer material has great potential applications in the next-generation high-performance spintronic devices, and further experimental verification and exploration of this material and related two-dimensional materials are needed.