We design a novel X-ray image detector by lens coupling a Gd2O2S:Tb intensifying screen with a high performance low-light-level (L3, which often means luminescence less than 10^(-3) Lux) image intensifier. Different coupling effects on imaging performance between zoom lens and fix-focus lens are analyzed theoretically. In experiment, for designing a detector of 15-inch visual field, the system coupled by zoom lens is of 12.25-lp/cm resolution, while the one with fix-focus lens is 10 lp/cm. The superiority of zoom lens is validated. It is concluded that zoom lens preserves the image information better than fix-focus lens and improves the imaging system’s performance in this design, which is referential to the design of other optical imaging systems.