Abstract: The critical elastic solution of stiffened penstocks under external pressure is calculated based on the Mises theory to study the buckling characteristics of a penstock with composite lining and partial drainage board laying. Then, we consider variations in constraint strength, material nonlinearity, and the non-uniformity of the surrounding medium, and develop a three-dimensional numerical model of composite lining based on ABAQUS. This model is applied to a comprehensive study on the critical instability characteristics of the penstock under the local constraints of the surrounding medium, and comparison is made with the tunnel exposed penstock scheme and the underground penstock scheme. The results show that the constraint effect of self-compacting concrete improves the penstock’s external pressure resistance, while the existence of drainage board reduces its resistance. The Mises solution adopted in the standard is suitable for the stability design of penstocks under external pressure with fullcircle external constraints. After the local drainage board is laid, the penstock’s critical buckling pressure is close to that of the tunnel exposed penstock scheme, both showing multi-wave instability. Thus, for the penstock with composite lining, we recommend the constraint effect is not necessary to be considered in its buckling stability design.