To improve the self-field critical current density (Jc) and critical temperature (Tc) in SiC-doped MgB2, optimization of the nominal Mg/B mixing ratio has been performed. The effects of the nominal Mg/B mixing ratio (x:2) on the superconductivity, Raman spectra, current transport properties, and flux pinning mechanisms in MgB2 with 10 wt.% SiC doping were investigated systematically, with x varied from 1 to 1.2. It has been found that the sample with the Mg/B ratio of 1.15:2 exhibited the best Jc performance for all fields (0–8.5 T), and its Tc was also enhanced. The optimized Mg/B ratio diminished the interband scattering caused by Mg or B vacancies, and in turn, increased Tc. The connectivity and disorder were increased in the Mg1.15B2 sample. Both of these were responsible for the improved Jc under all the fields examined.