Anaerobic fermentation tests were carried out with 20% solid content at the temperature of 55℃ to investigate the effect of hydrothermal pretreatment on thermophilic anaerobic digestion of food waste. Orthogonal test L9 (3³) was employed to determine the effects of water addition ratio, temperature and time. The results show that the physical and the chemical properties of food waste change significantly after hydrothermal treatment. The daily biogas production, the cumulative biogas production and the removal rates of TS and VS of food waste increase significantly after pretreatment. When the water addition ratio is 50%, temperature is 120℃ and pretreatment time is 80 min, we get 101 050 mg/L SCOD, which is 5.6 times of that of the untreated group. The peak values of daily and cumulative biogas production also reach the maximum under these pretreatment conditions. The maximum cumulative biogas production is 269.10 mL/g VS, which increases by 49.4% than that of the untreated group. The influences of the three factors on thermophilic anaerobic fermentation of food waste are temperature>time>water addition ratio. The effects of temperature and time of hydrothermal pretreatment on food waste fermentation are evident, while the effect of water addition ratio is less evident.