The theoretical model of single-end etched fiber Bragg grating (FBG) for simultaneous measurements of surrounding refractive index (SRI) and temperature in the low refractive-index area (from about 1.333 to 1.360) is researched, the effects of the structural parameters on the SRI sensitivity and linearity are analyzed in detail, and the corresponding methodology of linear approximation and error analysis is established. Simulated results show that the SRI sensitivity can be increased by reducing the diameter of the etched region or selecting the FBG with a relative bigger grating pitch for the fabrication of the sensor, however, these will cause not only the decline in the linearity but also the rise in theoretical error of the SRI sensitivity of the sensor. Based on the theoretical analysis, a single-end etched FBG is designed and fabricated for the corresponding experiments, the results of which are in accord with the simulated ones.