In recent decades, with the level of material life improves quickly, more and more people are on fire for outdoor adventure. They usually trek in desert and wasteland. However the case that compass or communication equipment is out of order happens frequently. For someone who lack direction, it is easy for them to be lost. So it is a meaningful work to research a way to find position with the help of the sun shadow. The sun shadow position is a huge project. Our paper aims to solve one link of the whole project. As the time and location change, the length of pole's shadow change, which we make research on to get the exact function model. As is known to all, earth-sun distance is much higher than mean radius of the earth. So we can approximately think the sunlight shining on the earth is parallel light. If we believe pole length is constant. The shadow length only relates with solar elevation angle. So we can use solar declination angle, solar hour angle, geographical longitude and latitude to calculate solar elevation angle. In a word, we use parameters such as date, longitude, latitude and time to refer to above variable, with the help of geographical knowledge. And then we establish the model which describe the change of shadow length. At last we apply the model in practice. The result prove that our model is reasonable.
