This research presents a study on improving the heat transfer of a flat solar collector system for air heating using triangular fin attached on the bottom face of an absorber plate in solar collector using a numerical simulation. A three dimensional numerical emulation was attained utilizing a Computer simulation of fluid flow via CFD process established on a k-ε disruption representative for renormalized array to predict the heat transfer improvement of flat solar collector system for air heating. The numerical results acquired into a constant thermal flux 1000 W/m² and the rang of Reynolds number is from 2349 to 9311 that were validated by comparing them with a practical data obtainable in the scientific published works. The numerical outcomes revealed that the use of triangular fins can effectively influence the thermal boundary layer by inducing periodic flow recirculation behind the fins. The numerical results also showed that at a Reynolds number of 5000 a maximum Nusselt number of 26.0 can be reached using a 10 mm fins but at the cost of a significant increase in the friction when applying a 15 mm fins. Consequently, the thermo-hydraulic coefficient of performance (THPF) for 10 mm fins was found to be greater than 0.88 for all Reynolds numbers, making it a better choice for a solar air heating system.