The invention discloses a plastic external corner angle bead for an inner wall of a building and a construction method for internal and external corners of the inner wall of the building. The plastic external corner angle bead comprises a surface type veneer external corner angle bead or coating type veneer external corner angle bead for protecting the external corner of the inner wall in construction, wherein the surface type veneer external corner angle bead and the coating type veneer external corner angle bead are both arranged from front to back along the length direction of the protected external corner; the cross section of the coating type veneer external corner protector is Y-shaped and the external corner angle bead comprises a corner protector lath arranged on the protected external corner as well as a first pressure strip and a second pressure strip which are respectively connected with the inner end of the corner angle bead lath; the surface type veneer external corner angle bead comprises a corner angle bead strip and a third pressure strip arranged on the corner angle bead strip; the cross section of the corner angle bead strip is of a sector shape. The construction method for the internal and external corners comprises the following steps of: (I) base layer cleanup; (II) construction on the internal and external corners. The internal and external corner angle bead disclosed by the invention is simple in structure and reasonable in design; the construction method for the internal and external corners has simple steps and reasonable design, is convenient to realize and easy in control over the construction quality, and can be used for solving multiple problems of an existing construction method for the internal and external corners of the inner wall of the building.