The construction phase of sports buildings is characterized by high carbon emission intensity, yet existing studies have largely focused on operational energy consumption and static life-cycle accounting, with no systematic investigation of the dynamic interactions among labor, materials, machinery, and environmental factors during construction. To fill this gap, this study develops an agent-based modeling (ABM) framework for a university natatorium in Shaanxi, China, to assess carbon emissions during the civil engineering construction phase and analyzes the independent and synergistic effects of labor, materials, machinery, and environmental factors. The simulation results show that, among single factors, material recycling achieves the highest reduction efficiency (57.93%, under the avoided-burden approach, representing a technical upper-bound estimate), followed by labor skill improvement (3.08%) and machinery maintenance (0.26%), while adverse weather increases carbon emissions by 13.20%. Multi-factor synergy analysis reveals that labor skill improvement buffers weather-induced increases (synergy: +610.49 t, 4.26%), and the full-intervention scenario achieves a 52.33% net reduction under adverse weather, though weather impacts cannot be fully offset. The proposed framework provides methodological support and a decision-making basis for low-carbon construction planning of natatoriums and similar buildings.