Many studies have investigated the crude oil separation process's separation mechanisms, size, and design, employing horizontal three-phase gravity separators in depth. There are, however, very few articles on their dynamics, modelling, simulation, and control. Understanding its dynamic behavior will aid in designing and tuning the device that can manage water level, oil level, and gas pressure in response to feeding variations. This scientific project gives a complete mathematical analysis, modelling, and simulation of a crude oil separation process using a horizontal three-phase gravity separator using Mathworks Matlab R2016b-x64 and Aspen Hysys V10. Bishoy's equations, which were constructed, will assist in operating this gadget, locating various variables, and observing the effect of modifying variables on the system's variables. The rationale for this study was developed in response to the small number of articles discovered, which may be a covert issue held up by large oil companies, as well as the complicated equations related to this process that remain unsolved, and to monitor what is happening in this complex dynamic process.