Hydraulic fracturing technology is an important technique for developing tight oil and gas resources. Multicluster fracture diagnosis is significant for hydraulic fracture design and optimization. Water hammer fracturing diagnostic technology is an emerging diagnostic technology. Fracture network complexity can be assessed by the water hammer decay rate. In this paper, a new characteristic line coupling model for the fracture-wellbore system is proposed. This model calculates the number of fractures and perforations based on the water hammer signal. First, our paper defines a method for calculating the water hammer decay rate. Second, a characteristic line simulation method for water hammer coupled with fractures and perforations is proposed. Third, the effects of different fracture-wellbore systems on the water hammer decay rate are analyzed. The method is validated by laboratory experiments and numerical analysis. Finally, through a field case, the results of the number of fracture openings explained by the optical fiber and our model are compared to provide support for the field water hammer fracturing diagnosis. Our paper provides a new model for water hammer decay rate calculation and evaluation of fracture number.