In order to get the simulation models of organic materials in the molecular simulations,based on the model establishment of a single molecular chain using methods of "random walk" and "directional walk",a method to build the simulation cell of organic materials in molecular simulations is brought forward by adopting the principle of uniform distribution of all molecular chains in the whole cell.Then the cubic simulation cell with periodic boundary conditions and the nano thin film with free surfaces can be built by using this method simply.Two representative examples and the comparison of the obtained results with those obtained by other methods show that the proposed method is simple,convenient and reliable.