A new type of sensor based on a doubly cladding single-mode optical fiber is proposed. The measurable range of the refractive index changes for the sensors is analyzed theoretically and demonstrated experimentally, which corresponds to the case that the fundamental mode in the fiber becomes leaky. Based on the analysis for the theoretical and the experimental results, the principle of the sensor is got. The output of the optical power of the sensor is almost inversely proportional to the refractive index of the outer cladding in some range of variation after the base mode in the fiber becoming leaky. A sensor with a linear response in the temperature range of 55~105 ℃ and an optical power variation of 39 dB is obtained. A new method of recoating hydrogels on optical fibers is verified. A humidity sensor fabricated by agarose shows a power variation of 9 dB in the moisture range of 30%~100%RH and insertion loss of 0.5 dB.