As global energy infrastructure like transmission and transformation projects expands, environmental pressures—particularly wastewater pollution—during construction are rising, yet understudied. This research systematically characterizes pollutant profiles in wastewater from various construction scenarios—including substation foundation excavation, transmission line foundation excavation, mechanical equipment flushing, and dewatering—across three Zhejiang Province projects (Huzhou Changxing 500 kV, Ningbo Hangzhou Bay 500 kV, and Nanxun 220 kV). Samples were analyzed for pH, turbidity, total suspended solids (TSS), and oil content, and evaluated using a Comprehensive Pollution Index (CPI). Results indicate neutral to slightly alkaline wastewater (pH 7.4–7.9); TSS ranged from 41 to 181 mg/L, and oil content from 0.51 to 1.97 mg/L. While simple physical settling (up to 3 h) reduced turbidity by 26.7–72.7%, it effectively failed to reduce TSS or oil concentrations, suggesting the presence of fine, hard-to-settle particles and emulsified oil forms. Compared to typical construction site runoff documented internationally, observed TSS was lower, but oil levels were comparable. This study fills a critical gap by detailing these scenario-specific wastewater characteristics and demonstrating the inadequacy of single-stage sedimentation for effective remediation. The findings provide essential data for crafting scientifically grounded, multi-stage pollution control strategies tailored to the unique demands of transmission and transformation project sites.