The utilization of chlorine-based ballast water treatment systems is essential for preventing the spread of invasive species, making the continuous monitoring of total residual oxidant (TRO) levels imperative for compliance with environmental regulations. This study developed and optimized a phosphate buffer-sulfuric acid-based solution (PSS) for use in the N, N–diethyl–p–phenylenediamine (DPD) colorimetric method to determine TRO in ballast water. Batch experiments demonstrated that pH 6 is the optimal phosphate buffer condition. Furthermore, the addition of iodide ions promoted rapid and stable TRO measurements across various conditions including pH and ammonium concentrations, sodium hypochlorite (NaOCl) concentrations, temperature, suspended solids (SS), natural organic matter (NOM), and water types. The optimized PSS achieved TRO measurement accuracy within ±5% of reference values, notably surpassing the commercial citrate buffer-p–toluenesulfonic acid-based solution (CTS), especially under challenging conditions such as low NaOCl concentrations, higher temperatures, and the presence of SS. PSS exhibited an average error rate of 3.16%, compared to 4.44% for CTS. Field tests at a ballast water treatment facility further confirmed the reliability of PSS for real-time TRO monitoring. This optimized reagent provides a practical, cost-effective solution for ballast water treatment systems, supporting environmental protection and regulatory compliance.