Reduced-scale models are often established based on similitude theory as an alternative to the direct experimental observation on the prototype, which is usually oversized or requires unacceptable expenses. Much insight into the similitude theory applied to various fields in structural engineering, vibration and impact problems has been gained to date. However, the efficient dynamic similarity design of complex rotors remains elusive. This paper is devoted to developing a reduced-scale model based on similitude theory from a high-speed rotor system prototype. Three critical speeds within the range of operating speeds characterize this flexible rotor. A reduced scaling design strategy for the complex rotor system is proposed as a two-step scheme. Similarity conditions relating the critical design parameters (such as rotor geometry, support stiffness, etc.) between the reduced-scale model and the prototype are derived. The scaling factors are accordingly determined by a dimensional analysis in combination with the governing equation of rotordynamics. This leads to a downsized rotor model with distorted geometric configuration whose operation speed is efficiently narrowed down. Dynamic similitude is assured by proportionally scaling down the three critical speeds while the rotor mode shapes still maintain high correlation between the prototype and downscaled model. The resultant reduced-scale model of the rotor system will practically guide the construction of the essential part of a whole engine dynamics test rig for laboratory use.