In this paper, different pressure boundary conditons are applied to the lubricating oil film in a piston ring pack. A variety of numerical methods are proposed and film thickness calculating values are compared with measuring values. The comparison demonstrates that in a complete working cycle,for a piston ring pack under both fully lubricating and starved lubricating condition, the calculating values of oil film thickness under half Sommerfeld's boundary condition are closest to the measuring values and it requires less computing work.