Based on the modified Rytov method, a model of scintillation index of echo wave from the point target in slant atmospheric turbulence is proposed. The model extends the scintillation index from weak fluctuation regimes to the moderate and strong fluctuation regimes in both monostatic system and biastatic system. Besides the inner scale, the model also contains the outer scale which changes with altitude. Numerical analysis results show that the outer scale affects the scintillation index little in the weak regime but plays an important role in the strong regime. The results above can be used for forecasting the influence of atmospheric turbulence on laser communication systems or laser detection systems.