In low-Reynolds-number flow past circular cylinder,the shape of vortices in the vortex street is not ideally axisymmetric,and changes with the the flow downstream. To solve the problem,finite element method was used to simulate the flow and the character of the vortex shape was analyzed by vorticity contour. Through analysis of a few contours,the vorticity contour changes due to the difference between the vorticity contour and the pressure contour. Generalized Joukowski transformation was used to describe the deformation of the vortex from axisymmetry.The result shows that three parameters describing the vortex change in respective ways,namely elllipticity varies parabolically, eccentricity varies linealy and bending angle is nearly constant. Kinematic expressions corresponding the three parameters were brought forward,comparison of the two expressions is satisfactory.