The hydraulic balance of farmland water conservancy pipe network is a common problem in the current large-scale farmland irrigation system. It is of great practical significance to study the different types of unsteady flow hydraulic calculations in farmland water pipe network for the safe and stable operation of farmland irrigation. In this paper, based on the characteristic’s method, the mathematical model of the unsteady flow for farmland water pipe network was established, and the boundary conditions of the constructed model and the processing method for calculating the time step were given. Then, the proposed model was verified by actual examples, and the unsteady flow process of the irrigation pipe network was also calculated and analysed. The research results show that the method of characteristics can better optimize the boundary problem of water pump, nozzle and t-branch pipe in the unsteady flow of farmland irrigation pipe network. After the valve is closed, the water hammer pressure will appear in the irrigation pipe network system and propagate inside. The water hammer pressure at the valve is the largest; the greater the distance from the closing valve, the smaller the water hammer pressure. When closing multiple valves at the same time, complex water superimposing of hammer wave occurs in the pipe network, and the maximum water hammer pressure after superimposing is much larger than the single valve closing. The longer the valve is closed, the smaller the maximum head pressure at the same monitoring point. The research conclusion can provide a theoretical reference for the engineering treatment of the unsteady flow in the farmland water conservancy pipe network.