Large-pipe-diameter large-flow measuring system and method

The invention provides a large-pipe-diameter and large-flow measuring system and method. The large-pipe-diameter and large-flow measuring system comprises a main pipeline, a branch pipeline and an ultrasonic flowmeter, an inlet and an outlet of the branch pipeline are connected with the main pipeline, and the ultrasonic flowmeter is installed at the position where the distance between the main pipeline and the center line of an inlet tee joint of the branch pipeline is larger than or equal to 7 d; the distance between the main pipeline and the center line of the branch pipeline outlet tee joint is not less than 3d; and according to the invention, a bypass branch pipe is additionally arranged parallel to the axis of the main pipeline, the calibrated high-precision ultrasonic flowmeter is used for measuring the flow of the branch pipeline, the proportionality coefficient of the flow of the branch pipeline and the flow of the main pipeline under different working conditions is calculated in combination with a nu