Two unexpected experimental phenomena in transmitted and reflective heterocore optical fiber sensors are observed. The surface plasmon resonance (SPR) wavelength of the reflective SPR (R-SPR) sensor redshifts about 100 nm to the transmitted SPR (T-SPR) sensor. The maximal extinction ratio is 0.60 for the T-SPR sensor and reaches 0.77 for the R-SPR sensor. For a further understanding of the generation mechanism of SPR in both sensors, the finite-element method is employed to calculate distributions of the electric fields. Accompanied with experiments, modal energy coupling among the modes as light transmits through the multimode-single-mode-multimode fiber sensor is demonstrated, and the length of the single-mode fiber (SMF) is the focus of interest. The initial dissipated energy of the higher-order TM modes breaks the dynamic coupling balance of modal energy in the SMF when SPR takes place as a significant perturbation. With increasing SMF length, extra energy couples from lower-order modes to higher-order modes, from TE modes to TM modes as well, which results in a deeper trough and a longer SPR wavelength and thus a more sensitive device. The results are promising for various applications of a broader spectrum, such as modulators and sensors.