Abstract: Analyzed here in the paper is the point of view as mentioned in the conventional welded pipe shaping theory that says that the baseline of the uphill shaping is not suitable for shaping of the light-wall pipe.It is pointed out that it is biased to simply use the plastic extension limit theory to explain the longitudinal edge instability of the shaped pipe blank so as to get the above mentioned conclusion.Furthermore with the pressure bar instability principle,the longitudinal stresses of the blank pipe edges concerning the uphill process,horizontal process and the downhill process are compared to identify that edge of the blank pipe as shaped with the uphill process is able to always maintain sufficient longitudinal tensile stress instead of compressive stress.It is the longitudinal compressive stress that acts as the root cause for edge instability of the light-wall blank pipe.Described at last here are the new type baseline of the uphill shaping process,and the role it plays in preventing and eliminating the instability during the shaping process of the light-wall pipe.