Plastic film is impermeable. Because of evaporation of soil moisture and transpiration of crops, the relative humidity of air in greenhouse increases. Without ventilation, the relative humidity of air in greenhouse can reach 90% or even 100%. The change rule is that the shed temperature rises and the relative humidity decreases; the shed temperature decreases and the relative humidity increases; the relative humidity in sunny and windy days is low, and the relative humidity in cloudy and rainy days is increased. The absolute humidity in greenhouse increases rapidly with the increase of greenhouse temperature, even doubles. If the relative humidity in the greenhouse is too high, the transpiration of plant leaves will be inhibited, soil humidity and air humidity in the greenhouse will increase, and a large number of water droplets will condense on the greenhouse film, which not only affects the photosynthesis of crops, is not conducive to the growth and development of plants, but also makes the reproduction of pathogens, disease epidemic and serious harm. Therefore, regulating temperature and humidity in greenhouse, especially at night, is an important measure to control diseases and promote crop growth and development, and should be paid attention to and strengthened.