The responsive characteristics of the structured fiber Bragg grating (FBG) under liquid medium with the refractive-index (RI) linear distribution are analyzed. The simulated results show that the reflection spectrum of the structured FBG are closely related to the properties of the RI distribution function of the liquid medium, such as the RI gradient along its axis and the value of the RI at both of its ends. Based on the numerical results, the theoretical model for the linear-approximation measurement of the RI gradient of the liquid medium by the structured FBG at the low RI area (1.330~1.360) is established. Through the experiments, the feasibility of the theoretical simulations and analyses and the practical applications of the RI gradient sensor based on the structured FBG are verified.