When optical wave propagates through the turbulent atmosphere,it is well known that the theoretical results of phase fluctuations under the Rytov approximation are not in agreement with those in practice.In this paper,the authors proposed that the Andrews Modulation Approximation could be used to solve this problem.Based on the principle of energy conservation,the scintillation modulation approximation theory was expressed in its explicit forms.Then by replacing the power spectrums of scintillation variances,the expressions of phase fluctuations of three links,uplink,downlink,and horizontal,were obtained.It is found out that the Andrews Modulation Approximation has many different forms,and for each form,its parameters have different physical meanings.In comparison with the results under the Rytov approximation,the results by this method in this paper are suitable for the situations,such as slant propagation,and scintillation convergence and saturation.