In this paper, the multi-disperse systems composed of 2048 frictionless particles with different size polydispersities are simulated by using the distinct element method, and effects of size polydispersity on mechanical and geometrical properties of these systems are studied under isotropic confining. The result shows that changing polydispersity can qualitatively affect the coordination number, shear modulus, static structure factor and orientational order correlation function, but it does not influence the cumulative force distribution. At small wave number k, static structure factor of mono-disperse systems shows the power scaling law S(k)∝0.2k-4/3. Moreover, the peak of orientational order correlation function exhibits an exponent scaling law g6 (r)∝ae-r/ξ6, and the scaling exponent ξ6 decreases with polydispersity increasing.