The invention belongs to the technical field of lithium ion battery materials and manufacturing processes thereof, in particular relates to an in-situ synthesis Li2MnO3 coated and modified lithium ion battery cathode material and a synthesis method thereof, and aims to solve the problems of the ordinary lithium ion battery cathode material, which is short in cycle life, low in capacity, narrow in charging and discharging voltage window, serious in side reaction in a charging and discharging process and the like. The in-situ synthesis Li2MnO3 coated and modified lithium ion battery cathode material disclosed by the invention is composed of a Li2MnO3 coating layer and a lithium ion battery material. The method of the in-situ synthesis Li2MnO3 coated and modified lithium ion battery cathode material disclosed by the invention comprises the following steps of: (1), configuring solution; (2), pre-processing materials; (3), performing in-situ synthesis of a MnO2 coating layer; and (4), performing high-temperature solid-phase reaction. The cathode material prepared by the invention is used in the field of lithium ion batteries.