An energy adjusting method for controlling output energy of a fuel cell group, wherein at least one of the fuel cell group and a secondary battery group drives a load is disclosed. The voltage of the fuel cell group is boosted by a boost regulation module to generate a first adjustment voltage. The boost regulation module boosts the voltage of the fuel cell group according to a first control signal. The first adjustment voltage is dropped by a drop regulation module to generate a second adjustment voltage to the load. The drop regulation module drops the first adjustment voltage according to a second control signal. At least one of the fuel cell group, the boost regulation module, the drop regulation module and the load is detected to generate a detection result. The first and the second control signals are generated according to the detection result.