The invention discloses a protection device for a combiner box, wherein the combiner box comprises a photovoltaic cable and a circuit breaker. The protection device for the combiner box comprises an acquisition unit, a logic unit and an execution unit, and is characterized in that the acquisition unit is electrically connected with the photovoltaic cable and used for acquiring voltage signals of the photovoltaic cable; the logic unit is electrically connected with the acquisition unit and used for receiving the voltage signals transmitted from the acquisition unit and comparing the voltage signals with comparison voltage, and the logic unit sends an instruction to the execution unit when an absolute value of the difference of the voltage signals and the comparison voltage is greater than or equal to a set value; and the execution unit is electrically connected with the logic unit and controls the circuit breaker to open according to the instruction transmitted from the logic unit. The protection device for the combiner box can effectively control the circuit breaker to open when the photovoltaic cable is directly grounded, so that a series of problems caused by grounding of the photovoltaic cable are solved.