The stress analysis method for fixed tubesheet (TS) heat exchangers (HEX) in pressure vessel codes such as ASME VIII-1, EN13445, and GB151 all assume a geometric and loading plane of symmetry at the midway between the two TSs so that only half of the unit or one TS is need to be considered. In this study, the midplane symmetry assumption is discarded. More common situations are considered such as unequal TS thickness, different edge conditions, pressure drop, and dead weight on two TSs. Based on the classical thin plate and shell theoretical solution, an analytical method of stress analysis for TS is presented. The proposed method is suitable for different types of HEX due to fewer assumptions employed in this study. Analysis shows that floating and U-tube HEX are the two special cases of the proposed method. Theoretical comparison shows that ASME method can be obtained from the special case of the simplified mechanical model of the proposed method. Typical geometries and loading are considered, and the proposed method is used to check the adequacy of design. Predictions are compared with the results obtained from axisymmetric finite element analysis (FEA) and current ASME method. Comparison results indicate that predictions given by this paper agree well with FEA while ASME results are not correct or not accurate.