This paper try to apply the variational technic to the statistical forecast field and some experimentsare performed with MOS equations of the highest temperature in Lanzhou city during Septemberto November 1999.The results show that the new method can improve the forecast and the results are related to MOS forecast ability.To explain it more,we propose a new concept,model skill(MS),and find that MS is steady for all forecast time when having optimized the MOS equations with the new method.