The 600m tunnel drilling machine is driven by hydraulic. In the working process, the hydraulic system generates a lot of heat which will be radiated by radiation system. According to heat output power, fancooled radiator was preliminarily chosen, and effective radiating area of the radiator slice was 25m2. In this paper, the function of heat change with time was derived and by the functional changes and extremum, radiator efficiency of hydraulic system was verified.