The optical effect of the steady droplet on the horizontal plane was studied.When a collimated-beam illuminated upon the symmetric droplet surface form below in a vertical direction,the steady and visible diffraction pattern was observed experimentally,and the relationship between the intensity distribution of the pattern and the illuminated area covered by the beam was found.While the diameter of the beam changes from larger to small,the intensity distribution of the pattern changes,either.The disappearance of the zero-order diffraction fringe,which corresponds to the 100% efficiency,was also observed in the experiment.Based on the physical optical theory,the analytic expression of the intensity distribution of pattern was derived,which explained the disappearance of the zero-order fringe well,and the height of the droplet at the certain position was also obtain.A novel method for the axisymmetric liquid surface deposited on the horizontal plane was found.