In this paper, core-cladding modal beating in a short piece of all-solid photonic bandgap fiber (AS-PBF) is observed in longitudinal propagation direction. It is demonstrated that at the stopband range of AS-PBF, the power could transfer back and forth between the fiber core and the first layer of high-index rods. Both experimental results and the theoretical analysis from transverse coupled mode theory confirm that the 3-dB width of the sharp stopband could be significantly narrowed by multicycles of such core-cladding modal couplings, which is of great benefit to the high-resolution sensing applications. Based on such a guiding regime, a high-temperature sensor head is also made and its response to temperature is tested to be of 59.9 pm/°C.