Adsorption of an oil-proof agent (OPA）onto cellulose fibers during the papermaking process was investigated using UV-Vis spectrophotometry by measuring the OPA concentration decrease in the solution. From the calibration curve, the spectrum were converted to chemical concentrations in solution, from which the amount adsorbed onto the fiber surface could be determined. Thus, it was possible to determine the total amount adsorbed onto the fibers and in solution. Using this approach, we studied the adsorption behavior of the OPA onto the fiber surface and derived its, ${Gamma}^s_t={Gamma}^s_{infty}(1-e^{-k_{alpha}t})$. The values of the parameters kaand ${Gamma}^s_{infty}$ were determined using a mathematic model based on a mass transfer equation. Ultimately, a complete was derived: $Q={alpha}{cdot}sumlimits_{i-1}^m{pi}d_il_i{cdot}M_A{cdot}(1-e^{-k_{alpha}t})/A_N$.