The optimization of the operating pressure of the separators in the multistage crude oil production units has an undeniable effect on the quantity and quality of oil production. In this regard, the present study exploited a simulation-based approach to optimize a multistage crude oil production unit through determining the optimal separator pressure and number which maximizes the oil production rate, and operational flexibility while minimizing fixed and operating costs, and power consumption of the compressors. The decision-making process was performed for two cases in the National Iranian South Oil Company. The number of separation stages and their different arrangements were considered as the desired goals. According to the results, for the first case, maximum oil production can be achieved using these two-phase separators and one degasser tank, while the cold stripping method was recommended for the second case. Furthermore, economic evaluations were conducted by calculating the fixed initial investment and the total operating costs. The simulation results predicted the pressure of the production well in 2030 as 8.27 MPa. For the reservoir pressure of 7.58 MPa, the fixed project costs will be reduced by $11965307, while the oil production will decrease by about 20 barrels per day. It will result in a $58.4 million reduction in revenue over the next twenty years. Therefore, the optimal pressure of the reservoir was assumed to be about 6.89 MPa.