L/W coexistence zeolite was synthesized with low-cost silica-alumina microspheres via an in situ hydrothermal method and modified by Mg as catalyst additive for FCC gasoline hydro-upgrading. The physicochemical properties of the supports and corresponding catalysts were characterized by means of XRD, SEM, N-2 adsorption-desorption, NH3-TPD, Py-IR, H-2-TPR, HRTEM, and Raman spectroscopy. The characterization results demonstrated that the L/W composite possessed the typical characteristics of L and W zeolites and that the morphology of L/W coexistence zeolite was completely different from that of the mechanical mixed zeolite. NH3-TPD and Py-IR results showed that the amount of acid sites decreased with an increase in Mg incorporation. Moreover, H-2-TPR profiles indicated that Mg regulated the metal-support interaction, which was conducive to the formation of type-II CoMoS phase. With the promoting effects of Mg, Cat-2Mg catalyst exhibited a good balance between hydrodesulfurization (HDS) efficiency (91.4%) and the preserving ability of the octane number (the decrease in research octane number (Delta RON) = 0.8 units), which could be attributed to the synergy effects of proper texture property, suitable acidity, and moderate MoS2 slab distribution.