Due to the wide range of applications of plasmonic diffraction gratings, it has become essential to provide an analytical method for modeling performance of the devices designed based on these structures. An analytical technique, in addition to greatly reducing the simulation time, can become a useful tool for designing these devices and predicting their performance. However, one of the major challenges of the analytical techniques is to improve the accuracy of their results compared to those of the numerical methods. So, here, a modified transmission line model (TLM) has been presented for the one-dimensional grating solar cell considering diffracted reflections in order to improve the accuracy of TLM results. Formulation of this model has been developed for the normal incidence of both TE and TM polarizations taking into account diffraction efficiencies. The modified TLM results for a silicon solar cell consisting of silver gratings considering different grating widths and heights have shown that lower order diffractions have dominant effects on the accuracy improvement in the modified TLM, while the results have been converged considering higher order diffractions. In addition, our proposed model has been verified by comparing its results to those of the finite element method-based full-wave numerical simulations.