An ultrashort and high-sensitivity refractive index sensor based on dual-core photonic crystal fiber (PCF) is proposed, and the characteristics are evaluated by finite-element method. The coupling characteristics can be changed by filling different index liquids in the holes of the PCF. Also, the transmission spectrum also changes as the length of the optical fiber is set to a specific value. The average sensitivities of (refractive index unit) and are realized in the index range from 1.41 to 1.438. The corresponding linearities are 0.9936 and 0.9916, respectively. The fiber length is just 3 mm, which could be widely used in the integrated sensing system. The sensor with high sensitivity and nice linearity is significative.