10:30～14:30为宜。The effects of the air velocity in heat exchanging pipes to heat and water vapor transformation was conducted in double-film covering greenhouse and the rational running mode was presented. The temperature and humidity in inlet and outlet of heat exchanging pipes, floor temperature in heat storage greenhouse and outside temperature were measured at different air velocities in the heat exchanging pipes. The humidity ratio, enthalpy of inlet and outlet of heat exchanging pipes and the power for heat saving were calculated. The results showed that the enthalpy of hot air decrease apparently after flowing through heat exchanging pipes when the air velocity in heat exchanging pipes was respectively 0.6m/s, 1.0m/s, 1.5m/s, 2.0m/s, 2.5m/s and 2.8m/s during heat saving in clear day. In addition, the air enthalpy difference between inlet and outlet of heat exchanging pipes and its variation tendency were similar at different air velocities. The power for heat saving enhances with the increase of the air velocity in heat exchanging pipes. The power and the increase of floor temperature held lower when the air velocity in heat exchanging pipes was less than 2m/s during heat storage. The appropriate air velocity in heat exchanging pipes was 2.5m/s to 2.8m/s, and the rational time for heat saving was 10:30 to 14:30.