A series of MgxB2 (x=0.9, 0.95, 1.0, 1.05, 1.10) bulks samples were prepared by the in-situ solid-state reaction method, and then the effects of Mg/B stoichiometry on superconductivity and flux pinning properties of MgB2 were discussed. Phase purity, microstructure and superconducting properties were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and magnetic properties measurement system (MPMS), respectively. The results show that the sample with a nominal composition, Mg0.95B2 has the highest Jc value among all the samples. In the Mg-deficient MgB2, the degraded grains grow and lack of Mg sites are responsible to significant enhancement of the Hirr and Hc2 and the enlarged △Tc and FWHM, resulting in the great improvement of flux pinning properties of MgB2