The magneto-electro-elastic Eshelby S-tensor is the key to the study of linear effective properties of magneto-electro-elastic composites. There are eight different ways to write the constitutive relations, and each is associated with a specific kind of boundary condition and Eshelby S-tensor. In this work, we provide a general procedure to convert the magneto-electro-elastic Eshelby S-tensor from one system to another. As an application, we use it to calculate the magnetoelectric coupling coefficients of a piezoelectric–piezomagnetic multiferroic composite under stress-and strain-prescribed boundary conditions. We demonstrate that the calculated results are significantly different. In particular, it is shown that, under an applied stress, the magnetoelectric coupling coefficient [Formula: see text], is much stronger than that under an applied strain, while for [Formula: see text], the values are positive under a prescribed stress but negative under a prescribed strain. The effects of inclusion shape, volume concentration and geometrical exchange, are also examined. For ready applications, the explicit forms of S-tensor, [Formula: see text] and [Formula: see text], of 1-3 fibrous and 2-2 multilayer composites are also provided at the end.