The invention discloses a hydraulic system for jacking a top cover in loosening the reactor top cover. The hydraulic system comprises an oil tank, a filter, a hydraulic pump, a first three-position four-way solenoid valve, an oil distribution device and a plurality of hydraulic cylinders. The oil distribution device comprises an oil distribution flow passage and oil cylinders as many as the hydraulic cylinders. An output port B of the first three-position four-way solenoid valve and cavities below inner plungers of the oil cylinders of the oil distribution device are communicated with the oil distribution flow passage. A hydraulic cylinder hoisting oil control pipeline is disposed between an upper oil access of each oil cylinder and an output port B of the first three-position four-way solenoid valve. Each hydraulic cylinder hoisting oil control pipeline comprises a three-position four-way solenoid valve which is communicated with the first three-position four-way solenoid valve, the oil distribution device and the hydraulic cylinders. Automatic control is achieved during reactor top cover loosening, labor intensity is lowered for operators, time for staying at a radioactive area is reduced for the operators, work efficiency is improved, and levelness of the reactor top cover during loosening is guaranteed.