The intelligent reconstruction of conventional substation needs to add process level device and to reconstruct bay level device so that causes complicated working and uneconomic for medium and low voltage bay. A distributed configuration scheme of secondary devices using intelligent electronic device（IED） with integrated functions for process level and bay level is presented. Based on the new scheme， a new type of integrated IED is designed， which completely implements merging-unit， intelligent-terminal， measurement-control and protection functions in one physical apparatus. The integrated design scheme and software-hardware architecture of the integrated IED is described， which is composed of four dominating parts: PowerPC main processor modular， digital signal processor（DSP） data processing module， field-programmable gate array（FPGA） integrated processing module and breaker intelligent control module. Several related issues are discussed in detail， including the design of sampled value data stream， rapid processing of generic object oriented substation event（GOOSE） and the design of measurement-control and protection function. Finally， the information model of the integrated IED is established according to IEC 61850 standard. In conclusion， the integrated IED， which is cost effective with reasonable design and superior performance， can effectively simplify the intelligent reconstruction of conventional medium and low voltage substations.