采用变分累积展开方法 ,研究了立方格点上磁性薄膜的热动力学性质 .计算自发磁化强度、内能和热容到了三级累积展开 ,并对每一级给出了这些物理量对薄膜原子层数的依赖关系 .虽然变分累积展开的收敛性还没有严格证明 ,但计算结果显示这些物理量的变分累积展开收敛很快 .三级计算结果足已表明 :对低于某一临界厚度的薄膜 ,自发磁化强度随原子层数的减少而减小 ;不论在临界温度以下 ,还是在临界温度以上 ,每单位格点的内能都随原子层数的减少而增大 ;每单位格点的热容在临界温度以下随原子层数的减少而增大 ,但在临界温度以上随原子层数的减少而减小 The variational cumulant method is used to study the thermodynamic properties of the magnetic thin films on the cubic lattice points.The spontaneous magnetization, internal energy and heat capacity are calculated to three levels of cumulative expansion, and for each level, Although the convergence of variational cumulants has not been strictly proved yet, the calculation results show that the variances of these physical quantities begin to converge rapidly, and the results of the third-order calculation have shown that for the convergence below a certain critical thickness The spontaneous magnetization decreases with the decrease of the number of atomic layers. The internal energy per unit lattice increases with the number of atomic layers, both below the critical temperature and above the critical temperature. The heat capacity increases with the decrease of atomic number below the critical temperature but decreases with the decrease of atomic number above the critical temperature