Magnetoelectric (ME) composite as one kind of ME material that can yield a strong coupling effect between magnetic and electric fields at room temperature, has drawn widespread attention for decades due to its rich physics contents and significant technological prospect. Except for traditional magnetostrictive/piezoelectric based ME composites, other ME composites have been reported, among which the magnet/piezo-cantilever composites show super strong ME coupling effect. The magnet/piezo-cantilever composite is generally composed of a piezoelectric cantilever and magnets attached at the free end of the cantilever, which realizes ME coupling by force moment-mediated magnetic torque effect and piezoelectric effect. Recently, various configurations of the magnet/piezo-cantilever composites for obtaining higher ME coupling coefficients have been proposed and demonstrated experimentally. However, few theoretical researches of these magnet/piezo-cantilever composites of different configurations have been carried out, which is of great importance for optimizing the design of ME coupling property of the magnet/piezo-cantilever composites. Here in this paper, a theoretical expression for the low-frequency ME coupling coefficient in the magnet/piezo-cantilever composite is deduced based on piezoelectric constitutive equations by using the theory of elastic mechanics. The typical magnet/bimorph composite is chosen as the theoretical model. Based on the deduced theoretical expression, the dependences of the lowfrequency ME coupling coefficients in the magnet/bimorph composite on material and structural parameters are numerically calculated. The results show that there are optimal thickness values of the piezoelectric layers in the magnet/bimorph composite with different metal thickness values and material constituents for achieving maximal lowfrequency ME coupling coefficients. The thicker the metal layer in the magnet/bimorph composite, the less insensitive the low-frequency ME coupling coefficient to the thickness of the piezoelectric layer will be. And the low-frequency ME coupling coefficient of the magnet/bimorph composite decreases when a metal with higher elastic module is selected for bimorph. For the magnet/bimorph composite consisting of hard piezoelectric ceramics (PZT-4), the low-frequency ME coupling coefficient is higher than that of the composite consisting of the soft counterpart ones (PZT-5 H), which is due to the hard piezoelectric ceramics with higher piezoelectric voltage coefficient than the soft counterpart ones. What is more interesting is that when the piezoelectric material in the magnet/bimorph composite is changed into relaxor ferroelectric single crystals Pb(Zn1/3 Nb2/3)O3-PbTiO3 (PZN-PT), an extremely high low-frequency ME coupling coefficient can be obtained, which is 3.8 and 5 times those of the 13 composites with hard and soft piezoelectric ceramics, respectively. This research gives a theoretical guidance for optimal design and practical applications of the magnet/Bimorph composite.