Abstract: Based on the conceptual framework of“three horizontals and four verticals”in EI（energy internet）,the development process of IES（integrated energy system）is divided into four stages in this paper:nurturing stage,conceptual stage,sailing stage and sublimation stage. In each stage the key technologies and typical forms are elaborated. The key technologies are cascade utilization,adaptation to local condition,multi-energy complementary and energy interconnection,respectively. Correspondingly,the typical forms of the above four stages are the combined cooing heating and power system driven by natural gas,distributed energy system,multienergy system and EI,respectively. Thus,IES is a multi-energy suppling situation,which includes cooling,heating,power and gas,and is driven by energy transformation and inter-crossing. Then the“commodification”of the shared energy in IES is gradually embodied under the internet technology. Meanwhile,the related programs,standards and demonstration projects of IES are coming into an advance trial phase. Finally,from the viewpoint of entropy,the development mechanism of IES is elaborated,where energy flow and information flow will be integrated profoundly and extensively. Namely,ubiquitous energy system has become the development direction of IES.