An optical surface plasma wave resonance (SPR) sensor based on the built-in modulation layer structure is studied. A kind of photoelectric composite film with unique performance is constituted by coating optical transparent films with different thicknesses and properties between the gold film and the fiber core, as internal modulation layer. It can adjust both the evanescent wave vector and gold film surface plasma oscillation wave vector, and then control resonance effect to provide a basis for sensitivity adjustment. Numerical simulation is done on the attribute of built-in modulation layer structure optical fiber SPR resonance incentive model using the finite difference time domain method. On this basis, a built-in modulation layer type optical fiber SPR sensor probe is developed for liquid refractive index measurement. The experiment results show that as the refractive index increases, the SPR resonance spectrum shifts to the long wave direction, in refractive index range from 1.335 to 1.392. The sensitivity can reach to 2263.1 nm/RIU, which is two times of conventional optical fiber SPR sensor based on the fiber core-gold film-environment medium three layer structure. All show that this new sensor can meet the needs of the environment index test much better.