Abstract: S shape curve(sigmoid) has an application in many fields.Most of the previous models contain only 3 parameters and take the symmetric and smooth S shape curve as fitting object so that it these fitting models can not be adapted to other more real situations: asymmetric and zigzag S shape distributed data.Therefore,author has created a "2 order rational data fitting model" which contains 5 parameters so that obtains more freedom for fitting the S shape distributed data.The Fitting experiments have clearly expressed that the suggested method obtains more advantages and can well fit different S shape curves considerably.