In this paper, we report a sensitivity-controllable refractive index (RI) sensor based on a reflective θ-shaped microfiber resonator cooperated with Vernier effect. The θ-shaped microfiber resonator is a reflective all-fiber device with comb spectrum under weak coupling condition. By cascading it with a fiber Fabry-Perot interferometer, Vernier effect is generated to demodulate surrounding RI with enhanced sensitivity. Theoretical analysis reveals that RI sensitivity of the combined structure with Vernier effect is m times higher than the sensitivity of singular θ-shaped microfiber resonator. Moreover, by adjusting cavity length of the θ-shaped microfiber resonator, magnification factor M = (m + 1) can be tuned which enables the RI sensitivity to be controlled. Experimental result demonstrates that the RI sensitivity can be widely tuned from 311.77 nm/RIU (Reflective index unit) to 2460.07 nm/RIU when the cavity length of the θ-shaped microfiber resonator is adjusted from 9.4 mm to 8.7 mm. The θ-shaped microfiber resonator based all-fiber RI sensor featuring controllable sensitivity and compact size can be widely used for chemical and biological detections. The proposed scheme of generating Vernier effect also offers a universal idea to increase measurement sensitivity for optical fiber sensing structures with comb spectrum.