Total-internal-reflection (TIR) typed fiber sensors based on photonic crystal fibers (PCFs) are made by filling samples into PCF cladding holes, where the interaction of light wave occurs between the evanescent wave of fiber core and the filled samples. This can avoid common transmission losses caused by fiber surface roughness. The interaction region of the evanescent wave in a PCF and the filled samples are almost coincident, so increasing fiber length can enhance the light-matter interaction and enable accurate detection of tiny changes of samples. However, it is difficult to inject sample materials into stomatal cladding of TIR-typed PCF sensors due to small volume of pores. In addition, the energy utilization rates of TIR-typed sensors are relatively low, only about 6%. A main influencing factor on the sensitivity of TIR-typed PCF sensors is the power fraction of pores in the whole cross section. In order to improve the sensitivity, one can elevate the power fraction of PCF pores. Based on the above considerations, a novel three-core double-clad PCF is designed, where samples are injected into the middle hole and two Yd-doped cores are arranged on its two sides for active excitation. Our theoretical calculation and experimental test show that this kind of structure can not only increase the coupling efficiency of the evanescent wave into the air holes effectively, but also gain higher detection sensitivity of trace samples.