Abstract: Tip-tilt indetermination with Laser Guide Star (LGS) is one of the important factors to limit the sky coverage for laser guide star adaptive optics (LGS-AO). Measuring and correcting the angle-of-arrival fluctuations of laser uplink is an effective method to solve this problem. This paper proposes a method to measure angle-of-arrival fluctuations of uplink laser, which works without natural guide star and auxiliary telescope. The main idea of that method is that LGS is detected by Shack-Hartmann wave front sensing and the laser uplink angle-of-arrival fluctuations is reconstructed by the data from selected sub-apertures. In this paper, the relationship between the error of the optimal algorithm and the number of selected sub-apertures is simulated. The results show that the decline ratio of the minimum algorithm error relative to the telescope's full-aperture tilt error is not related to r₀, but is related to the telescope's aperture. The larger the telescope aperture is, the more the algorithm error decreases. When the telescope diameter is 10m, the minimum algorithm error drops to 33% of the telescope's full diameter tilt error.