We investigated the gate-tunable spin transistor structured as a ferromagnet–topological insulator–ferromagnet junction implemented on a zigzag phosphorene nanoribbon. By applying a gate voltage to the device region, the wave vector of the channel can be effectively modulated, thereby shifting the resonance condition of the junction and enabling efficient tuning of the spin-dependent conductance. The conductance of the junction can be switched between e^2/h and 0, corresponding to the ON and OFF states, respectively. Our work demonstrates a feasible strategy for realizing spin transistors and highlights the potential of such systems for future spintronic applications.