With density functional theory and nonequilibrium Green's functional method, we studied effects of overlap area on spin transport properties of zigzag graphene nanoribbon (ZGNR) junctions. It shows that the system exhibits semiconductor transport properties. The spin up and spin down currents are nondegenerate after adsorption of graphene nanosheets. With the decrease of overlapping carbon atom chains, the transport ability is decreased. However, as graphene nanosheets are adsorbed on the edges of ZGNR spin transport properties of the system are the best, due to the excellent expansibility of spin up and spin down of Homo+1, Homo and LUMO. It provides a theoretical reference for the preparation of graphene circuits and devices.