This paper reports a single cell refractometer consists of FBG fiber-based resonant cavity by employing buffer modulation method. In this biophotonic chip, single cell is trapped optically in the cavity, and the transmission spectral shifts due to the change of surrounding buffer, without and with cell, are used to determine the cell's refractive index (Rl) and size. The measurement shows that MDCK cell has an effective Rl of 1.383 at a precision of 0.1%. Moreover, the measurement method with optical trapping eliminates uncertain factors and prevents cell damage. The chip is promising for label-free cell measurement and further cell analysis.