Spent caustic is one of the most harmful wastes of refineries, gas and petrochemical industrial processes. Considering the CCD optimization methods, various experiments were conducted to neutralize the spent caustic  and then using the design expert software, the process parameters including initial temperature and reaction time were considered as effective parameters. The COD neutralization process was modeled as a linear equation. The main finding of the paper is that using the sulfuric acid neutralization method for spent caustic treatment, the COD of the spent caustic was reduced from a high value of 34,000 ppm to 8,000 ppm. All sources of harmful components of the spent caustic were converted to sulfate salts. The industrial limit for spent caustic wastewater is that the COD value should be in the range of 60-100 ppm. However, this value is difficult to achieve and is usually achieved by diluting the final spent caustic wastewater. pH changes during the neutralization process started from a maximum value of 14 and was reduced to 12 by neutralizing the free caustic values.. According to the explanations given, the COD amount in different stages ranges from 34,400 ppm to about 20,000 in the neutralization stage (pH 7) and to 8,000 in the acidic stage (pH 2).By neutralizing the above materials, the pH value decreases from a high value of 12 to 7 and finally to 2.