Strength calculation for tubesheets in floating-head heat exchangers (FHXs) in current standards is based on analytical theory which assumes that the two tubesheets are symmetrical with same thickness, same material. This assumption results in a design that is not optimized for the requirement of the FHXs. In this paper, a new analytical theory is proposed which is able to perform strength calculation for tubesheets of different size and materials in FHXs. Comparison of the analytical stresses with those obtained by finite element simulation shows that the new theory proposed here is reliable and applicable. In addition, this new theory covers the symmetric one adopted in current standards such as ASME and is effective for the optimization design of the floating tubesheet (FT) as well as other relevant elements in the FHXs.