Abstract: Analysing stress and strain in the frame of solar plastic greenhouse is the necessary means to assure normal function. The distribution and the maxmum value of stress and strain is directly related to the shape of the frame. Optimizing the shape to minimizing the peak value of stress and strain is the high way to design the greenhouse economically. In this paper, a model to analyse the stress in frame was established. With the optimizing principle, a parabola frame was seen as the best in the 7 groups of curves. By the model, we can get optimized rest of any size of plastic greenhouses withh an inner post.