Abstract: Studied on are the phase transition behaviors of the 38.5 mm X80 heavy pipeline-purposed steel plate,and the influence by the cooling process on its microstructure.As a result it is revealed that under the cooling condition of low average cooling rate+low final cooling temperature,the micro structure of the whole thickness of the heavy steel plate becomes finer and more homogeneous,and the steel also has the yield ratio as ≤ 0.85,and better DWTT toughness.Furthermore the formation capacity and the manufacture feasibility of the UOE production line of Baosteel are evaluated with digital simulation method,based on which the industrial trial manufacture of the Φ1 422 mm×38.5 mm X80 ultraheavy pipeline-purposed steel plate and the UOE weld pipe are fulfilled.All the property indexes of the steel tubes from the said trial production are in conformance with applicable technical requirements as specified under API Spec 5L.