A simpler numerical model for plate-fin heat exchangers with plain fins and serrated fins is presented, which incorporates the characteristics of fluid flow and heat transfer. The numerical simulations for heat exchangers with two fins at low Reynolds numbers are carried out by employing the simplified model and using the CFD code FLUENT. The results of heat transfer rate and pressure drop in numerical simulations are compared with the experimental results from the literature. It is shown that the results are in good agreement and the numerical model is reasonable and appropriate. The characteristics of fluid flow and heat transfer are analyzed and compared based on the numerical results. It is concluded that by applying the average heat transfer coefficient in practice, the heat transfer rate of the primary surface is overestimated, and the heat transfer rate and efficiency of the secondary surface are underestimated. The numerical modeling and conclusions provide the method and theoretical basis for the selection, optimization and analysis of plate-fin heat exchangers.