The sun glint observation mode is adopted when a satellite payload detects greenhouse gases over the ocean to obtain high-precision gas concentration data. Therefore, accurately calculating the real-time sun glint position is a prerequisite for obtaining high-precision data. An improved sun glint calculation model based on an optimal line search is proposed. According to the WGS-84 ellipsoid model, on the basis of dichotomy searching to meet the requirements of specular reflective speckles, the algorithm is improved by using the idea of optimal line search to better meet the coplanar condition of the sun glint. Compared with the simulation results of the Satellite Tool Kit (STK), the sun glint position distance error calculated by the improved model is significantly reduced to 90 m.