A new theoretical model about magnetron sputtering is suggested to study thickness uniformity of films. In this model we obtain an explicate formula on the film thickness, and its approximate expression at the third order in Taylor's expansion. According to the formula, we have drawn the three-dimensional curves of film thickness versus the positions of that film. From the curves, the optimum positions of the film can be easily obtained for preparing uniform films or depositing films rapidly. Finally, a discussion about similarity and difference between planar and cylindrical magnetron sputtering is given.