A series of Mg/Ca modified HZSM-S zeolite catalysts were prepared by the impregnation method for catalytic coupling of CH3Cl to light olefins. The catalysts were characterized using X-ray diffraction, N-2 adsorption desorption, NH3 temperature programmed desorption, and pyridine-IR techniques. Results showed the Mg/Ca modification substantially reduced the strong Bronsted acid amount and greatly improved light olefin selectivity. The Mg-rich catalysts were more inclined to generate C3H6 and C4H8 due to less strong acid sites but shorten the lifetime due to smaller pore size. The 5Ca2Mg-HZ catalyst offered the best performance with 87.2% total olefin selectivity and 99% CH3Cl conversion over 58 h. Coke analysis revealed all the modified catalysts had similar coking behaviors while the short-lived catalysts showed an early drop in surface area owing to the blocked channels. The deactivation regeneration experiments showed the SCa2Mg-HZ catalyst also had good reproducibility.