A new method of horizontal tank tilt ratio based on cloud data by laser scanning is presented. Horizontal tank can be supposed as approximate cylinder. The equation of the cylinder axis can be gotten by fitting the cloud data. First, pretreatment method based on the curvature algorithm and the symmetry principle was presented to separate line segments and arc segments from single layer data. The interference of attachment points had been excluded through linear least squares fitting in order to get some stable cylindrical fitting parameters. Then, Levenberg-Marquardt nonlinear least-squares method was applied for fitting to get cylinder parametric equation. Finally, horizontal tank tilt ratio was obtained by exactly calculating. Compared with manual measurement, experimental results show that this method has fast computing speed and strong robustness, and angle deviation was less than 0.01 degrees.