In order to study the correction method of instrument coefficient of cylindrical gear meter that works in hydraulic oil under variable pressure conditions, the gear flowmeter is calibrated at different pressure conditions with the flow standard facility which works with hydraulic Oil. Two methods of fitting with binary surfaces are used to fit the calibration results, and the error of the meter coefficient after corrected by two methods at each condition is less than 0.4% by the fitting correction, and it’s sure that the correction methods are useful.