According to the common mode of production and the aim of research on watermelon in plasticcovered greenhouse in Dongtai, a meteorological station is built. Samples of both low temperature in the nighttime and high temperature in the sunny daytime are obtained respectively. After the data sequence is studied and analyzed, it is concluded that there is a close relationship between the temperatures in and outside the plasticcovered greenhouse at nighttime, and under the sunny conditions, the temperature inside has significant correlations with the external temperature, the solar altitude and the slant distance of sunlight through the atmosphere. Relationship, which is established by using the statistical regression analysis method, between the temperatures in and outside the greenhouse under both the cold conditions at nighttime and sunnyday conditions, has higher precision or reaches the practical precision. Low temperature ranges of alert levels at nighttime are defined with the temperature index of freezing injury and possibility. A tentative forecast is made on the time of occurrence of index temperatures inside the greenhouse in sunny days, and thus contributes to serve the watermelon production in plasticcovered greenhouse.