An optical detection system for measuring a refractive index of a liquid analyte comprising a light transmitting assembly fiber, which includes a single-mode fiber, an optical fiber sensing module and a multi-mode fiber. The optical detection system is configured to emit a coherent light beam to one end of the assembly fiber; and a detector is configured to capture a specklegram from an exit end of the assembly fiber. When the optical detection system is in operation, the optical fiber sensing module is configured to be submerged into the liquid analyte. By analyzing the correlation between the specklegram corresponding to the liquid analyte and the reference specklegram, the refractive index of the liquid analyte can be obtained.