The invention belongs to the technical field of power energy storage, and particularly relates to a system and method for changing power of a waterhead adjustment pumped storage unit through compressed air. The system is characterized in that a variable-speed pumped storage generating device B is connected with a high-pressure buffer pool D and a low-pressure pool C and connected with a power gridsimultaneously; a first hydraulic cylinder set of a self-adaption hydraulic potential energy conversion device A is connected with the high-pressure buffer pool D and the low-pressure pool C; a second hydraulic cylinder set of the self-adaption hydraulic potential energy conversion device A is connected with an external liquid potential energy source E; and the unit rotation speed of the variable-speed pumped storage generating device B is variable so that the unit operation power can be changed, the pressure difference of the high-pressure buffer pool D and the low-pressure pool C is changedthrough expansion and compression of gas so that a proper waterhead matched with the rotation speed is formed, and the total per area of the two hydraulic cylinder sets is changed by the self-adaption hydraulic potential energy conversion device A so that the potential energy difference of the high-pressure buffer pool D relative to the low-pressure pool C and the potential energy difference of the high-pressure buffer pool D relative to the external liquid potential energy source E can be matched.