Abstract: In order to realize refractive index (RI) measurement with low cost and high precision, a multimode fiber liquid refractive index sensor based on U-shaped microstructure was fabricated by femtosecond laser micromachining. The relationship of the variation of the optical power of the sensor, the depth of the U-shaped groove and the refractive index of the liquid in the U-shaped groove were studied. The influence of the different ablation lengths on the sensitivity was also discussed. The sensing mechanism was analyzed based on the influence of the radiation theory and the model. The results show that the sensor has a good linear response in the range of 1.3331~1.3731(refractive index), and can reach 5700μW/RIU (refractive index unit) sensitivity. While in the case of 10dB loss, a good sensitivity can be obtained at 20μm ablation length. The sensor has advantages of simple structure, easy preparation, high sensitivity and low cost. It has wide application prospect in chemistry, biology, medicine and environmental monitoring.