The three dimensional temperature field of the gap between piston and cylinder was obtained by numerically solving energy equation. The boundary condition of the equation was given in the form of heat transfer coefficient, instead of solving the temperature field of solid parts. The temperature field was calculated both under high speed high pressure condition and low speed low pressure condition. The numerical result was compared to experimental result under low speed low pressure condition and showed good agreement. It was shown that the influence of heat transfer coefficient on film temperature was significant. The adiabatic condition was reasonable under low speed low pressure condition, but invalid under high speed high pressure condition. It was a good way to describe the influence of solid parts on temperature using heat transfer coefficient but avoiding solving the temperature field of solids parts.