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Effects of the air velocity in heat exchanging pipes on heat exchanging and water vapor transformation was studied in double-film covering greenhouse during heating at night in winter. 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 heating were computed. The results showed that the temperature and enthalpy of cold air with high humidity flowing through heat exchanging pipes increased apparently and the humidity reduced when the air velocities in heat exchanging pipes were respectively 0.6m/s, 1.0m/s, 1.5m/s, 2.0m/s, 2.5m/s and 2.8m/s during heating at night after heat saving in clear days. The power for heating was enhanced with the increase of the air velocity in heat exchanging pipes. The average power for heating is 1.0kW, 1.6kW, 3.2kW, 6.4kW, 7.2kW and 7.7kW respectively. The heating effects is not distinct when the air velocity in heat exchanging pipes is less than 2m/s, while the continuity for heating is poor when the air velocity in heat exchanging pipes is higher than 2.5m/s. Therefore the rational air velocity in heat exchanging pipes is 2.0m/s for heating at night when the temperature satisfies the growing requirements of the crops.