An in-line Fabry-Perot (FP) refractive index (RI) sensor based on an intrinsic FP cavity and fabricated by the etching and fusion splicing method is proposed. The experimental results demonstrate that the sensor possesses a high resolution of 1508 nm/RIU for the measurement of acetylene gas RI. The temperatureresponse measurement shows that the sensor is insensitive to room temperature variations. The FP RI sensor is suitable for applications in biosensing and environmental monitoring because of its high sensitivity and structural simplicity, thereby making it suitable for low-cost mass production.