The principle of least action with variables was used to solve the forced vibration of the rectangular plate with three clamped and the other free under concentrated load, and the corresponding stable solution was obtained.Moreover,the principle was extended to solve the forced vibration of straight stem and bending of thin rectangular plates,and a new method for solving this kind of problems was established.The results were compared with the literates,which proved our method to be true.So the method gives us a new way to solve some problems about earthquake resistance and vibration analysis of the architecture engineering.