Phase structure function and aperture-averaged slope correlation function with finite outer scale are derived. The strong effects of finite outer scale on measurement of characterization of atmospheric turbulence statistics are shown, especially for large size telescope and subaperture. The phase structure function and atmospheric coherent length based on Kolmogorov model is just the proximation of the formalism derived here. The analysis indicates that it can not be determined whether the deviation from the model of Kolmogorov turbulence is caused by real variance of spectrum or by the effect of finite outer scale.