To address the limited adjustable capacity of distribution networks (DNs) under the large-scale integration of flexible resources into microgrid (MG), an MG flexible operation region (MGFOR) model is proposed to quantify the adjustability of MG under aggregated regulation cost constraints. A coordinated dispatch framework of DN and MG is then developed based on MGFOR to enable cooperative flexibility allocation. The original boundary of MGFOR is determined through a radial iterative linear search, and its construction is completed using a convex-hull fitting approach. The simulation is conducted on the modified IEEE 33-node test system. It is demonstrated that MGFOR explicitly quantifies adjustable power boundaries, yielding 2.2% lower total operation cost for the proposed coordinated dispatch framework of DN and MG compared with hierarchical dispatch framework of DN and MG. The voltage safety margins at noon are enhanced by up to 56.46% at distribution feeder terminals. Furthermore, the impact of operation modes of distributed photovoltaic units on node voltage in DN is comparatively analyzed.