This research adopts the research idea of “key analysis and general comparison”，selects 5 streets for field research，comprehensively analyzes the reform of the “large department system” of street government in Beijing，and puts forward countermeasures and suggestions for deepening the reform. The study found that the “real department system” of completely breaking small departments and the “imaginary department system” in which small departments and large departments coexisted are the two modes of the reform of the “large department system” in the street government. The reform of the “large department system” has enhanced the efficiency of system operation from two aspects：resource planning and internal coordination. In the actual operation of the “large department system”，there are problems such as the lack of deep internal organic integration，poor external coordination with top and bottom，unclear responsibilities and unclear levels of department-level cadres. The deepening of the reform of the “large department system” requires，on the one hand，to enhance the depth of integration within the large departments to release the reform dividends more thoroughly；on the other hand，to solve the new problems caused by the reform to enhance the external coordination efficiency of large departments and the incentive strength of section-level cadres.