To fundamentally prevent thermally induced synchronous vibration instability in high-speed cantilever rotors, this study proposes a design criterion that incorporates the Morton effect. The influence of heat generation due to frictional losses in sliding bearings is first analyzed, and based on the auxiliary support criterion for cantilever rotors, a static deflection criterion considering the Morton effect is derived. A three-dimensional transient thermal bending model of the high-speed cantilever rotor is then established. A dynamic mesh technique is employed to simulate journal whirling at varying amplitudes, enabling the acquisition of surface temperature distributions on the journal. Finite element models using both three-dimensional solid elements and one-dimensional beam elements are applied to calculate the rotor’s thermal deformation and dynamic characteristics. The thermally induced synchronous vibration instability of high-speed cantilever rotors is investigated under different rotational speeds and journal diameters. The results indicate that the rotor’s thermal bending deflection primarily decreases with increasing journal diameter below 3000 r/min, whereas beyond this speed, the deflection increases significantly with rising rotational speed. In practical rotor-bearing system design, the proposed static deflection criterion can be employed for initial estimation, followed by unbalance response analysis incorporating the Morton effect for rotors that satisfy the criterion, ensuring safe and stable operation of the high-speed cantilever rotor.