In this paper, a decomposition method for limit of radiation emission inside an electronic system is proposed to solve the problems based on 10kHz-18GHz electric field. As the model structure, hierarchical structure and overall radiation limit of the entire system to be designed are determined, such radiation emission limit could be decomposed to obtain the limits of each electrical module, PCB board, and component inside this system in order. Thus, the overall radiation emission limit of the entire system could be allocated to each micro portion appropriately, in order that all segments subdivided for analysis could meet their radiation emission limits and to reach better electromagnetic compatibility (EMC) performance inside the entire system.