Abstract: The relationship between the longitudinal dunension of an abutment and the recirculation length behind it is established based on the expennental data and the numerical results using a 3D model.The law of wall and the large coefficient method are used in the 3D model.The results show that the length of recirculation decxeases with rise in the ratio of longitudinal dunension(b) to lateral dunerrsion(D).The recirculation length is sensitive to changes in b/D when b/D is small, and becomes a constant when b/D is larger than 4, at which the results are equivalent to the solutions to sudden expansion prolr lens.A formula is proposed to calculate the length of recirculation for longitudinal dunension of an alxztment.