The invention relates to a full-automatic online calibrating device and calibrating method for a vehicle-mounted large-aperture flow meter. The full-automatic online calibrating device comprises two parts, namely a field vehicle-mounted component A and a control room component B, wherein a flow meter to be calibrated and the field vehicle-mounted component A are connected in series front and back in an oil product channel; the field vehicle-mounted component A comprises an oil product channel formed by fixing an oil inlet quick joint, a manual valve, a gas-removing filter, a standard flow meter, a check valve, an electric valve, an oil outlet quick joint and a connection hose, and connecting telescopic fluid pipes in series; and the control room component B is used for inputting associated signals of the standard flow meter and the flow meter to be calibrated, running a calibration control computer and outputting control commands, wherein the standard flow meter and the flow meter to be calibrated are connected by an electric cable joint, the calibration control computer is provided with calibration control software, and the control commands are outputted by the calibration control computer through communication of electric cables. The full-automatic online calibrating device and calibrating method for the vehicle-mounted large-aperture flow meter disclosed by the invention have the advantages that the real online calibration is realized, the flow meter does not need to be detached, the oil product is not leaked, the calibration time is short, the amount of oil for calibration is small, the calibration efficiency of an oil library oil discharge flow meter is greatly improved, and the calibration cost is reduced.