A method with a set of improved basis functions in vaouum-jellium region for Film-LAPW, method suited to jellium/slab model is proposed. The calculations for examination of this method, with three layers of W slab to simulate the W(100) surface covered with Cs overlayer is made. The results are more stabilized and accurate, and are much closer to experiments. The contribution weights of each vaccum basis function to the wave function is analysed.