The air-sea fluxes and sea-surface heat budget over the East China Sea during a cold air intrusion are analyzed based on the observation data from a research vessel. During the period of the cold air from 20:00 on 5 to 14:00 on 6 May 2017, the average momentum flux was 0.22 N·m-2. The mean values of the sensible heat flux and the latent heat were respectively 27.17 W·m-2 and 90.25 W·m-2, which were 2.8 and 1.1 times of the mean value in the entire observation period (20 April to 26 May 2017). The daily mean net heat flux was -12.73 W·m-2 and the sea was losing heat during the cold air. During the day time of the cold air intrusion, the sea surface gained the heat of 58.36 W·m-2. The sea-surface heat budget was mainly affected by the net radiant flux and latent heat flux. During the night time, the sea surface lost the heat of 156.89 W·m-2, of which the latent heat flux (sensible heat flux) was 99.79 W·m-2 (27.11 W·m-2), accounting for 63.61% (17.28%) of the total heat released from the sea surface. The heat was transferred from sea surface to the atmosphere mainly by the latent heat flux.