Aiming at the diagnosis technology of the horizontal well fracturing effect, a numerical fracture diagnosis method based on the water hammer effect is proposed. Because of the energy dissipation caused by the fracture to the pump shutdown pressure, the traditional water hammer mathematical model is difficultly to apply to the fracture diagnosis of horizontal Wells. In this paper, considering the influence of fracture on the water hammer effect, a mathematical model of water hammer in a horizontal well is established based on the equivalent circuit method, and the model is solved by the characteristics method line. The fracture parameters are deduced based on the characteristics of the water hammer pressure curve, and the variation characteristics of the water hammer pressure curve with different influencing factors are analyzed. The results show that the characteristics of water hammer pressure wave are affected by fracture morphology and construction parameters. and the amplitude and period of hydraulic pressure are changed by fracture volume, displacement and fracturing stage length. The water percussion model is used to carry out historical fitting to the field data, and the coincidence rate of the obtained fracture half-length and height with the microseismic monitoring data is more than 80%, which verifies the accuracy of the model and provideds on idea and method for the fracture parameter evaluation of horizontal wells.