In view of the shortcomings of the beam pumping units, major oil fields and research institute researchers carry out a large number research of HPU(Hydraulic Pumping Units). Unlike conventional mechanical engineering, HPU belongs to outdoor equipment, must bear poor working conditions, must subject to high temperature and low temperature environment; swabs frequently and works 24 hours continuously. Optimizations on hydraulic oil selection and system design are needed to meet these special conditions for HPU. Based on the main points of traditional hydraulic system, both the selection of hydraulic oil and system design, including heater, cooler and temperature control system, were discussed.