Abstract: For 12-ram-thick hot-rolled X70 pipeline steel strips coiled at 500 ℃, a coupled thermomechanicalphase transition model in the laminar cooling process was established by MARC finite element software, and then the changes in temperature field, stress, strain, volume fraction of phase transformations and warping degree in the duration of laminar cooling were calculated at two kinds of cooling water ratios. The results show that the maximum warpage of the strip edge is 21.84 mm because of different strain values of the upper and lower surfaces resulting from different cooling rates in the thickness direction in the early stage of water cooling at a water ratio of 1.25. The steel plate will gradually restore straight in the later cooling process. However, the warpage of the strip edge is still -9 mm at the beginning of coiling due to the plastic deformation in water cooling and the different bainite contents in the thickness direction at finishing cooling. Uneven cooling between the upper surface and the lower surface is the fundamental reason of warping. Under the condition of ensuring the performance of X70 pipeline steel, the residual warpage of the strip edge is only -0.58 mm at the beginning of coiling when the water ratio is adopted to be 1.58. The proper water ratio can greatly reduce the strip's transverse warping in the laminar cooling process.