Principle of measuring refractive index fluctuation in atmospheric optical turbulence by the usage of a carrier modulated fiber optical Mach-Zehnder interferometer is introduced, as well as the signal processing procedure. Experiments show that the noise level of the system C(2,n) variance is about 2.5×10^－16. Comparison of the results with that of the traditional micro-thermal anemometer in the real atmosphere shows good agreement in the values and trends, which indicates that the fiber optic turbulence sensing technique is already applicable.