The invention relates to the technical field of engines, and discloses a control method and system for assisting a diesel engine in vehicle-mounted method emission. The problem that due to the low exhaust temperature, the conversion efficiency of a post-treatment system cannot meet the requirement is solved. The method comprises the steps that according to engine parameters, the exhaust temperature needed value is determined; when the exhaust temperature measurement value is lower than the exhaust temperature needed value, the exhaust temperature difference value is determined; according to the engine parameters, the exhaust temperature needed value, the exhaust temperature difference value and the calculation constant, the position needed value of a throttle valve is determined; accordingto the position needed value of the throttle valve and the position measurement value of the throttle valve, the drive current needed value is determined; and according to parameters of a control motor and the drive current needed value, an air inlet throttle valve control motor is controlled to reduce the aperture of an air inlet throttle valve. The control method and system for assisting the diesel engine in vehicle-mounted method emission are applicable to the treatment process for increasing the diesel engine exhaust temperature.