Fermat's principle is a basic one in the geometrical optics，which means that the actual trajectory in the space depends on the extremum of the optical path.Based on the Fermat's principle，a new method is proposed to get directly the equation for the practical optical path in reflections and refractions at spherical boundary for the object point outside the principal axis，and then the object-image formula is also derived in the condition of axial ray and axial object.