The invention relates to a precursor of a lithium ion battery cathode material, a preparation method thereof, a cathode active material of a lithium ion battery, a lithium ion battery, and a continuous reactor, and belongs to the technical field of new energy materials. The preparation method of the precursor comprises following steps: mixing nickel salt, cobalt salt, manganese salt, doped metal salt (D salt), a precipitating agent, and a complexing agent to obtain a reaction system; carrying out reactions under stirring at a temperature of 30-80 DEG C in an inert gas atmosphere, wherein the pH is 10.0-13.0 during the reaction process, raw materials are fed into the system continuously, and reaction products are discharged continuously through overflowing; and aging the overflowed materials for 1 to 5 hours to obtain the precursor. When the granularity of overflowed materials reaches a set value, the reaction system is subjected to an ultrasonic treatment until the granularity of overflowed materials is lower than the set value. The obtained precursor has the advantages of uniform granularity and morphology, and large tap density, and is benefit for the preparation of a lithium ionbattery cathode material with excellent electrochemical properties.