The coupling of reaction and diffusion processes on catalytic surfaces leads to spatio-temporal heterogeneity in concentration. Understanding of this phenomenon is very important for better catalyst design and higher reaction efficiency. In this work, molecular dynamics simulations combing hard-sphere and pseudo-particle modeling are carried out to investigate the coupling between the reaction and diffusion near an isolated active site in 2D pores with a simple reaction model. The local fluctuation in concentration caused mainly by the reaction-diffusion coupling is observed and proved to be non-stochastic using quantitative parameters proposed in this work. The reaction factor R and diffusion factor D are defined to quantitatively characterize the reaction and diffusion performance, respectively. The results show that the fluctuation is weak when the process is reaction-limited (e. g., R/D is very small) or diffusion-limited (e. g., R/D is very large). When R/D is within a certain range, the strongest fluctuation appears. Besides, the diffusion factor D has a relatively larger effect on the fluctuation than the reaction factor R. Similarly, it is found that the highest overall yield of the pore is obtained only when R/D is within a specific range due to the reaction and diffusion coupling. It is also found that the local fluctuation must be considered when studying complex reaction processes with different reaction orders. The results are expected to be helpful for understanding the reaction and diffusion coupling and the complex reaction kinetics at the atomic scale, as well as for the design of catalysts and the improvement of catalytic efficiency.