The steam generator is an important equipment of the coolant pressure boundary in the primary circuit of the nuclear island. The assembly process of steam generators requires extremely high technological and cleanliness requirements. In this study, we designed and established a cleaning room to complete the assembly process of a certain steam generator, which could ensure the indoor temperature (≥16℃) and humidity (≤40%). In this cleaning room, internal components assembly, welding, flaw detection, final assembly and other processes could be completed. The main research work included: (1) Main design methods for the process layout, steel structure, retractable roof, air conditioning system, and assembly platform of the cleaning room were summarized; (2) Stress analysis was conducted on the steel structure, retractable roof, and assembly platform of the cleaning room using finite element method, and the indoor air flow was analyzed using computational fluid dynamics method. The cleaning room (built in an enterprise in eastern China) met the requirements of technology, strength and cleanliness, and has passed the acceptance test successfully. Our methods and findings may be helpful for similar engineering projects.