The understanding of chemical reaction dynamic is one of the ultimate goals of chemistry. For many-body sysytem, there are so many problems in calculating with \textit{ab initio} method. so it is efficient to use a semiempirical potential energy function. For the study of $C_2H_5F$ system, A semiempirical functional form of the potential energy surface is used for the reaction channel for $C_2H_5F-->C_2H_4+HF$. The potential form contains Morse-type function for the stretching potential and switching function for the change of C-C bond character and attenuation functions for making a new bond of H-f along the formation of the fragment. The potential energy parameters used in the switching function and attenuation functions are fitted to the transition state of \textit{ab initio}calculations.