Many magnetoelectroelastic devices are mechanically clamped during service, and the clamping may lead to pre-deformation, which will in return affect the magnetoelectroelastic behavior. In the present paper, a theoretical model is constructed to formulate the fracture problem of a clamped magnetoelectroelastic strip loaded magnetically or electrically. Two groups of multiple unequal-length cracks, parallel to the lateral boundaries, are assumed to exist in the strip. In the case of antiplane deformation, Green’s functions of the shear stress components are derived by the method of dislocation simulation, and then they are employed to construct singular integral equations (SIEs) for the cracks. The SIEs are numerically solved to get the stress intensity factors (SIFs). It is found that the SIFs are linearly dependent on the applied magnetic/electric field and the clamping-induced antiplane pre-deformation. In addition, parametric studies are performed to reveal the effects of geometrical parameters on the SIFs, and the crack shielding and interference are regarded as the underlying mechanisms for the fracture behavior.