The invention discloses a network system for providing mobile edge computing service and a service method thereof, so as to solve the technical problem of flexible deployment of MEC on a mobile communication network. In the C-RAN architecture, an SDN-based MEC controller is deployed, and an MEC server is deployed in the BBU. The service implementation step mainly comprises the following steps: setting a decision threshold; judging whether the time delay is sensitive or not and the like to determine a computing mode by an MEC controller; and giving a computing result through four computing modes of computing of a local MEC server, combination computing of a plurality of MEC servers, computing of a specific non-local MEC server and computing of a cloud center; and finishing all user MEC tasks through repeated executions. According to the method, the MEC service is realized; meanwhile, the remaining computational resources in the BBU can be sufficiently used, and the MEC network hierarchyis more concise, so that the management is facilitated, the data transmission efficiency is improved, the pressure of a core network is relieved, and the overall computing task time delay is reduced.The method can be used for flexible deployment of the MEC on the mobile communication network in the present 4G period, the 4G to 5G transition period and the 5G period.