An automatic butting and posture-adjusting positioning device used for large cylindrical components and with a coordinated motion function comprises at least two sets of positioning mechanisms which are fixedly connected with the to-be-assembled large cylindrical components through embracing hoops. Each positioning mechanism comprises a chassis, a transversal posture-adjusting platform serving asa y-direction moving pair, a rotating posture-adjusting platform serving as a passive rotating pair around a z-axis, a vertical lifting platform serving as a z-direction moving pair and a bracket usedfor supporting the embracing hoops, wherein the transversal posture-adjusting platform, the rotating posture-adjusting platform, the vertical lifting platform and the bracket are arranged on the chassis; and the rotating posture-adjusting platform is connected with the transversal posture-adjusting platform and a vertical lifting device, and the bracket and the embracing hoops are fixedly connected. The automatic butting and posture-adjusting positioning device has the advantages that the coordinated motion ability is high, the posture-adjusting range is wide, the butting precision is high, the overturn-preventing ability is achieved, and safety performance is high; and six-degree-of-freedom posture adjusting and precise butting and assembling of the large cylindrical components can be achieved.