To address the source-load power mismatch in off-grid photovoltaic (PV) hydrogen production systems arising from PV power fluctuation and the slow dynamics of alkaline water electrolyzers, a multi-mode coordinated control strategy based on real-time power comparison and multi-loop coordination is proposed. Firstly, the mathematical models of the PV array, a three-phase interleaved Buck converter, and the alkaline water electrolyzer are established. Then, a mode decision mechanism is designed to continuously compare the estimated maximum PV power with the temperature-adaptive safe power limit of the electrolyzer, thereby integrating electrolyzer thermal dynamics into the control loop for adaptive safety boundary adjustment. Furthermore, a power-voltage cascade controller is constructed to achieve smooth transitions between the maximum power point tracking (MPPT) and power-limiting modes, with a power overshoot of 1.48% and a mode-switching dead time of 0.3 s. Meanwhile, a current protection loop and a low-selector are introduced, suppressing the current overshoot from 14.8% to 4.96% under degraded main-loop performance. Simulation results demonstrate that under various disturbances, including irradiance variations and electrolyzer temperature rise, the proposed strategy effectively maintains source-load power balance, and ensures equipment safety, and achieves stable mode switching.