Abstract: Suitable time for uncovering and covering heat preservation curtain affects greatly the heat preservation of greenhouses and crop growth inside, especially in winter. Research on environment inside and outside of sunlight greenhouses in winter was done, according energy balance and method of regression, the formula to determine suitable time for covering and uncovering heat preservation curtain were derived under two typical weather conditions-sunny day and overcast day. Experiments were conducted to prove the correctness and practicality of the derived formula.