In order to understanding trapped-oil pressure and pressure difference of the oil-film both ends as well as especial fluid load how to impact on the minimum oil-film thickness in external gear high-speed lubrication pump. Firstly, from geometrical relationship of straight-tooth gear transmission and stiff-isoviscous lubrication theory, a new corresponding formula of minimum oil-film thickness included inlet pressure and outlet pressure and cracked-point position of oil-film were derived. All simulation results indicated that a large load fluctuations even negative load was caused by trapped-oil pressure; the minimum film thickness on the tooth surface was changed by the affected fluid load and the affected oil-film pressure difference by trapped-oil pressure, in general, the lubrication state of tooth surface would be improved because of the trapped-oil pressure and oil-film pressure difference; the elastic parameters and the minimum oil-film thickness in a full tooth surface cycle had a large change when trapped-oil pressure was presence; the stiffness theory could not be applied for the lubrication analysis of the tooth surface, And a suitable formula was selected to calculate the logical oil-film thickness in accordance with the actual elastic parameters and viscosity parameter,etc. Finally, the conclusion was summarized that the lubrication of the gear tooth surface in external gear high-speed lubrication pump was fully different from conventional gear, and relative lubricating theory derived from conventional gear must be corrected before applied to external gear lubrication pump.