Several methods of measuring C2n (refractive index structure parameter) are analyzed and compared, and the reason of leading this different measuring result is given. It is shown that the result of scintillation will change notably with power of temperature spectra when comparing with the result of double-point, but power of temperature spectra takes a little effect on angle-of-arrival variances and back-scattering of electromagnetic wave when comparing with the result of double-point. When outer-scale changes, the value of C2n with scintillation almost keeps equal, but when outer-scale decreases, the value of C2n with double-point and arrival angle will decreases very much. Especially, the relation between the double-point method and out-scale is verified, and the acceptable interval between two points is recommended.