Abstract: In order to solve the problem which the permanent magnets can be damaged for the huge centrifugal force at high rotation speed, the mechanical strength of high-speed permanent magnet rotor is researched. The huge centrifugal force is the main factor when the permanent magnet rotor rotates in large velocity. Protection bridge of the interior permanent magnet motor rotor sustain the centrifugal force which itself and permanent magnet causing. According to mass equality before and after, the equivalent ring method is put forward to deal with the rotor, and build mathematical model. The mathematical relationships between the thickness of the equivalent ring and the size of protection bridge are deduced. The results of the equivalent method and finite element method are contrasted in different speeds. The contrast results have shown that the equivalent method can calculate the stress effectively and accurately.