In this work, we report a novel electrolyte additive allyloxytrimethylsilane (AMSL) to improve the electrochemical performance of high voltage LiNi0.5Mn1.5O4 cathode. In presence of 0.5% AMSL, the discharge capacity retention of Li/LiNi0.5Mn1.5O4 cell is improved from 73.1% to 80.2% after 500 cycles at room temperature, and from 52.4% to 92.5% after 100 cycles at 55 °C. Moreover, the Li/LiNi0.5Mn1.5O4 cell with AMSL delivers a superior discharge capacity of 95.6 mAh g−1 at high rate of 3 C, whereas the cell without AMSL only remains 76.8 mAh g−1. Theoretical calculation and experimental results reveal that AMSL is oxidized prior to the carbonate solvents during the first charge process and then creates a less resistive and high thermal stable SEI film on the surface of LiNi0.5Mn1.5O4 cathode. The AMSL derived SEI film, composed of organic silicon-based species, ether moieties and reduced LiF, is responsible for the suppression of serious electrolyte decomposition and dissolution of transition metal ions at high voltage, especially at high temperature.