Abstract: The study aims to solve the solar shadow position.The relationship among the length of shadow and four parameters(longitude,latitude,date and moment)is deduced according to the calculation formula of solar apparent coordinate in spherical astronomy and the formation principle of solar shadow in this paper.Based on this,an optimization model which can be calculated particular date and geo-location is set up by adopting the least square method,and a search algorithm including rough search algorithm and exhaustive method was designed to calculate the model.The practice shows that the data which were measured at(79.50°E,39.87°N)was substituted to the optimization model.MATLAB program is used to solve the above algorithm,the result is(79.75°E,39.52°N),and the error is less than 1%.