The effect of outer scale of atomospheric turbulence on tracking precision of tilted tracking system is studied by considering G-tilt power spectrum and the residual variance of tilted tracking system. The G-tilt power spectrum and the residual variance of tilted tracking as functions of the outer scale of turbulence are deduced theoretically. The numerical results show that outer scale of turbulence has great effect on G-tilt power spectrum and residual variance of tilted tracking at low frequency but almost has no effect on the high frequency.