Abstract: The difference of solidification structure and macro quality between trail bloom and normal bloom has been comparatively analyzed for ø380~ø800 mm large round bloom. Some defects such as low ratio of equiaxed crystal zone,high ratio of white band,serious central porosity,cavity and crack are observed in the trail bloom.The causes have been theoretically analyzed based on running out process,and a capping delayed running out process has been put forward to improve the trail bloom quality.Industrial test result shows that the capping delayed running out process can delay the strand capping by adding some heating agent on the top surface and switching off the spraying water in order,which leads to more sufficient contraction compensation.Thus,unqualified ratio of trail bloom decreases from 6.25% to 1.96% when tail cropping length is reduced by 0.5 m,and the liquid steel yield is improved by 0.55%.