Abstract: Aiming at the weld-cracking problem in the process of using Tangsteel S380 to make welding-pipes,which happened in the tests of Flatten and Hole-expansion carried out on these pipes,microstructure-inspecting and property test were taken by the usage of SEM and microhardness tester. The results shown that Widmanstatten microstructures in the fusion-line and molten metal in the center of S380 weld are the main reasons causing the weld-cracking in the tests of Flatten. And then,some improving-measures were proposed for users and the rate of cracking had been greatly reduced.