The basic concepts and development trend of distributed generation（DG）， the micro-grid and smart distribution system are described. In particular， the influence of DG and micro-grid on the smart distribution system is analyzed from the point of view of the large-scale access of distributed energy resources. DG supplies users with green power from various locally available renewable energy resources， but large-scale interconnected operation of DG in the distribution system may give rise to security and operation problems. Micro-grid technology provides an interface to the interconnection of various DGs by means of structures at different levels， which is the most efficient way for the operation of DG. On the other hand， the smart distribution system makes it possible to realize flexible access to DG and safe， reliable， and optimal operation of the entire power system by effectively managing the micro-grids. Based on an analysis of the relationship between the three， suggestions about the idea and technical means are given for implementing the smart distribution system.