The invention relates to a reinforced foundation and a method for improving bending resistance of an original building rigid foundation. The reinforced foundation comprises the original rigid foundation and a bearing structure on the original rigid foundation, wherein a bending resisting steel member is arranged under the original rigid foundation, the length and the width of the original rigid foundation are expanded according to design calculated values, an interface agent layer is coated on the interface of a new foundation and an old foundation, a new expansion foundation wider and longerthan the original rigid foundation is poured in the root of the bearing structure and around the original rigid foundation, an under bending resisting steel member is poured in the new expansion foundation, and poured concrete is filled at the upper part of the original rigid foundation and a side-arranged reinforcement cage and poured to form a whole together with the new expansion foundation. The invention has the advantages of wide application range, low requirement on the original rigid foundation material, safety and reliability of the under bending resisting steel member, does not basically influence the normal life or work of residents or workers in the original building, is simple in operation, and can be widely applied to the building reinforcing reconstruction.