Abstract. Considering their particular geometric structures, the stator and rotor cans in canned motor reactor coolant pump are assumed to be elastic coaxial cylindrical shells in this paper. An ideal, incompressible, swirling fluid flows the annulus between the inner shell (can) and the outer shell (can). Thin shell structures such as cans are prone to buckling instabilities under fluid forces. Furthermore, a lot of accidents were caused by losing stability. Dynamic stability analysis of the stator can subjected to helical annular flow is presented by means of Donnell-type shell equations and the potential flow theory. Specially, the effects of the geometrical parameters of the annular flow-shell system are studied. The results show that the critical velocities decrease with the crease of the values of the length-to-radius and radius-to-thickness. Moreover, the effects of annular gaps and boundary conditions on the stability of annular flow-shell system are given.