The Gd substituted Y1-xGdxBa2Cu3O7-δ systems are studied systematically by positron annihilation and x-r ay diffraction. The x-ray results show that the crystal parameters and crystal v olume of the systems increase with the increase of Gd content. However, all the substituted samples maintain the single orthorhombic phase as YBa2Cu3O7-δ (YBCO) sample does. The positron annihilation result s show that positron lifetimes depend on the Gd content strongly. According to t he positron annihilation result, the local electronic density on Cu-O chain is c alculated. The result shows the local electronic density decreases with the incr ease of the Gd content, while the Tc increases with the decrease of t he local electronic density. The relationship between the local electronic densi ty and superconductivity is different from the substitution on Cu site, which ma y be one of the reasons why the mixed rare earth cuprate superconductor has a hi gher critical electric current density than the YBCO. These experimental results give the corresponding positron experiment data for the application of cuprate superconductors and study of the mechanism.