The transmission line of off-grid new energy generation hydrogen production systems is likely to cause system collapse due to faults, and the rapid detection and removal of faults is key to ensuring normal operation. To realize the full-range fault detection of off-grid hydrogen production system lines, and at the same time realize the coordination between the backup protection and the main protection, a cooperative protection method based on the fault instantaneous power differential rate of change as the main protection and the local power rate of change constituting the backup protection is proposed. First, the topology of off-grid hydrogen production system is established, and the hydrogen production grid is modeled. Secondly, the short-circuit fault currents and voltages are derived using loop analysis, and the instantaneous power differential rate of change of the fault is calculated. Finally, the positive and negative instantaneous power differential rate of change characteristics are analyzed with different types of faults. When the system communication is normal, the main protection is activated by comparing its fault instantaneous power differential rate of change with the threshold value, and then the post-fault power differential rate of change is used to construct the pole selection coordinate system to realize the fault pole selection. In the case of communication failure, the backup protection quickly removes the fault to avoid further expansion of the fault scope. This synergistic protection method has good speed, selectivity, reliability, and can cope with communication failure.