The invention relates to the technical field of a motor, in particular to a motor rotor structure and an assembly method thereof. The motor rotor structure comprises an iron core and a rotor assembly,wherein the rotor assembly comprises a rotation shaft and a bearing which are spliced and connected, the bearing comprises an outer ring, a rolling body and a bearing support, a circle of bearing inner channel is arranged on the rotation shaft, the rolling body is fixed between the outer circle and the inner channel by a bearing support, a key structure is arranged at one end of the rotation shaft, and a key hole is formed in the iron core and is matched with the key structure. In the motor rotor structure, the bearing inner channel is arranged on the rotation shaft to substitute an inner circle of the bearing, the rotation shaft and the bearing are integrated to form a rotor assembly, so that the problem of damage to the bearing due to a traditional process of pressing the rotation shaftto the bearing is solved; and moreover, the rotor assembly does not need to employ interference fit by matching of the key structure and the iron core, and the problem that the rotation shaft is bentand deformed when the rotation shaft is pressed into the iron core is solved.