The invention discloses a determining method for a flow rate weighting coefficient of each sound channel in a multi-channel ultrasonic flowmeter. The method comprises the steps of building a projection model of one single sound channel in the multi-channel ultrasonic flowmeter; on the basis of the single sound channel projection model, inferring a sound channel coverage region average flow rate formula; according to the Reynolds number and the tube wall roughness, calculating the flow rate weighting coefficient of each sound channel in the multi-channel ultrasonic flowmeter. The method is suitable for multi-channel ultrasonic flowmeters of different types, different specifications and different shapes and sizes, the purpose of determining the flow rate weighting coefficient of each sound channel in the multi-channel ultrasonic flowmeter is achieved, the cost of the multi-channel ultrasonic flowmeter is lowered, and application of the multi-channel ultrasonic flowmeter is promoted.