The thermal calculation of steam generators can be divided into two forms: one is the design calculation, which is mainly aimed at obtaining the required heat transfer area of the steam generator. This is under the condition that the structural form and some basic thermal parameters have been determined, to obtain the size of the heat transfer area. The other is the check calculation for existing equipment, where the heat transfer area of the equipment is known, and then some other thermal parameters are calculated based on some known thermal parameters to determine whether the equipment can meet operational needs. This chapter mainly introduces the design thermal calculation of steam generators.