In order to obtain the method for the failure mode determination of plain penstock with local wall-thinning，the failure mechanism of plain penstock with local wall-thinning was investigated.Firstly，the reasonability of the failure analysis method was proved based on the experimental data.Then，the volume ratio of plastic regions for plain penstock with local wall-thinning and intact penstock was selected as the quantitative index of failure mode.The relationship between the local thinned geometric parameters and the failure mode determination was proposed.The results show that the structural span and local thinned circumferential length have little influence on failure mode.It also indicates that when the local thinned thickness is no more than 0.5 times of the wall thickness，the control parameter of penstock failure mode is the local thinned depth，otherwise，the failure mode is controlled by both the local thinned depth and the local thinned axial length.