An isolated high (low) temperature hydraulic system was presented. The traditional oil temperature control technology of hydraulic system was improved. The hydraulic cylinder transfer system at room temperature was the front stage active, push/pull isolation cylinders were used for circulation work, the oil from the two isolation cylinders formed a closed system, the oil circulated, and it was heated or cooled through the oil temperature control device to reach the rated high (low) temperature. The advantage is that only temperature control of the oil fluid in the isolation cylinders is required,so the oil volume required to control temperature is significantly reduced, thus the power and complexity of the temperature control equipment are reduced. After the temperature balances, heating (or cooling) only needs to supplement the heat consumption, the operating power is small. This technique has achieved good practice result.