The invention discloses a method for measuring flow of a pipe network. A first receiver, a transmitter and a second receiver are arranged at the bottom of the pipe network and are arranged along the direction of the water flow rate of the pipe network, the first receiver and the second receiver are arranged at locations symmetrical to the transmitter, and a transmission direction of the transmitter is perpendicular to the direction of water flow. The method includes steps of uniformly dividing the cross section side of the pipe network into N portions along the transmission direction of the transmitter and computing the area of each portion; allowing the transmitter to transmit ultrasonic waves with the frequencies of f<0> in the transmission direction, allowing the second receiver and the first receiver to simultaneously receive reflective frequencies f<1> and f<2> of the ultrasonic waves in water, receiving included angles of the transmission directions where the reflective frequencies are located and reflective directions, multiplying the flow rate of the water by the area of each portion where the water with the flow rate is located to obtain water flow on the area of each portion, adding the water flow on the areas of the portions to obtain the water flow on the cross section side of the pipe network perpendicular to the transmitter. The ultrasonic waves in the water are transmitted by the transmitter.