Unsteady flow is an important engineering problem in urban pipe network systems, requiring pressure and flow rate management analyses and reliable drinking water quality maintenance. Efficiently solving the hyperbolic partial differential equation and integrating it with various boundary conditions under the complex layout scenarios of pipe networks is a challenging issue for pipeline modelers. Frequency-domain modeling with a time-domain response was developed as an alternative to the traditional method of characteristics. However, this solution requires a substantial array size for large pipe network systems, significantly affecting applicability in field pipe network systems. This study proposes an innovative transient analysis method, the modular impedance matrix method, to solve the most labor- and cost-intensive computational issues affecting the unsteady flow analysis of large, complicated pipe networks. This method was applied to a field pipe network system and its performance compared to existing approaches. The algorithm of the proposed method fundamentally solved the computational problems associated with other methods, and its modular scheme allowed feasible integration with an analytical formulation that can be tailored to the modeler's preferences. The modular impedance matrix method's strength can be amplified according to the size and complexity of the pipe network system owing to its unique complementary validation capability.