According to the poor working ability and unmanageable machining precision, an oversized bottom horizontally adjustable three rolls bending machine was successfully developed, which could process the steel plate of δ=250 mm thickness,b=3000 mm, Φ=6000 mm. Based on this bending machine, the structural characters and main technical parameters were discussed. Besides, the material mechanics were used to analyze the practical forcing with bending machine. Then, a mechanical designing model was set up. Meanwhile, the relationship between all main parameters was eventually obtained. The results show that this method can meet the strength and stability requirement, and the weight is reduced by 10.2%, which provides a very important theoretical reference for manufacturing such equipment.