The paper aims to improve the mechanical and hydrophobic performance of sodium alginate film. In this paper, n-octylamine (OA) was covalently connected to sodium alginate molecule through amidation reaction by using EDC/NHS as catalyst in order to improve the mechanical and hydrophobic performance of sodium alginate film. The mechanical properties of sodium alginate film were improved and its water solubility was reduced by the "association" between hydrophobic molecules. The grafting rate of octane amine and the film forming performance was investigated, and the effects of the addition amount of octyl amine on the mechanical properties, water solubility, water vapor transmittance, color and thermodynamic properties of the film were investigated. The molecular structure of octyl amine modified sodium alginate was confirmed by FTIR and 1HNMR. The structure of modified alginate film was analyzed by XRD and SEM. The results showed that uniform transparent films can be obtained when the grafting rate was less than 19.94%. Comparing to the pure alginate film, the strength (TS) and elongation at break of the modified film were increased by 23.1% and 35.5%, respectively. The thermodynamic data showed that the thermodynamic stability of the film increased with the increase of residual amount at the final heating temperature of 700 ℃ and the decrease of the maximum decomposition rate temperature. Comparing to native alginate film, water content, water solubility and water vapor transmittance of the OA modified alginate film decreased by 37.2%, 17.5% and 81.3%, respectively. It is an effective strategy to improve the physical and chemical properties of film materials through covalent modification to introduce long hydrocarbon chain into sodium alginate.