Against the backdrop of increasing attention from countries around the world to energy conservation and emission reduction policies, the design of water supply, drainage, and firefighting systems in construction projects has become an important aspect of improving energy utilization efficiency. This article takes the existing building water supply system as the research object, analyzes the energy consumption law of the existing water supply and drainage fire water system through comprehensive simulation and optimization methods, and establishes an optimization model for the existing building engineering water supply and drainage fire water system. Firstly, this article constructs an energy consumption database for building engineering water supply, drainage, and fire water systems. By using data-driven methods, the energy consumption and efficiency of each system under different working conditions can be simulated. On this basis, this article adopts genetic algorithm to optimize the system structure and achieve optimal energy efficiency. Effective reduction of rainfall runoff and improvement of water resource recovery and utilization efficiency can be achieved through the introduction of technologies such as rainwater harvesting and recycling, and Low Impact Development (LID). In the energy-saving performance test results of residential buildings, after six months of operation, the optimized system saved a total of 15,500 kWh of energy consumption. The research results will significantly reduce building water and energy consumption, and achieve industrial energy conservation and emission reduction goals.