The existing section function for mining subsidence prediction has several shortcomings, such as complex expression, uncertainty of coordinates origin and parameters in application. The features of observing data in the ground subsidence of rectangular goaf are studied and a new fit function of main section is proposed for mining subsidence prediction.This new function has some merit that the mathematical formula is simple and the parameter is also easy to identify in practices. It is proved that the results of this new function is fit well with onsite survey .