This paper presents an interferometric method for measuring the dry/wet mass of a single living cell using an optofluidic chip. It consists of the fiber Mach-Zehnder interferometer and the fiber-optical trap, both of which are integrated onto a single chip. In experiment, a single living cell is captured by the fiber-optical trap, and then the cell’s refractive index and diameter are simultaneously determined by the spectral shift in response to the buffer modulation. Hence, both cell dry and water masses can be calculated with a precision higher than 5%.