A theoretical model of sound transmission of an infinite orthogonally stiffened laminated plate was developed to investigate the sound insulation performance based on classical laminated plate theory, taking into account the effects of the bending and torsional movement of the stiffener. Then, by using the space harmonic expansion and the principle of virtual work, the sound transmission loss formula was derived. By comparing the predictions of the presented model with existing results of the simplified model and the platebeam combined model, the validity and feasibility of the proposed model was qualified. Finally, the influences of laying mode, angle, stiffness of layers, material loss factor and spacing of orthogonal stiffeners on the sound insulation performance were investigated by numerical calculation.