We investigate the structural changes of a polydisperse system undergoing an oscillator shear flow by use of a nonequilibrium molecular dynamics simulation. The nonequilibrium phase diagrams are investigated in the space of the amplitude of the shear rate and the number density at several polydispersities. At small polydispersity, phase changes to two types of layered structures are observed and this phenomenon is similar to that in the monodisperse case. With increase of polydispersity, the system becomes disordered, and an amorphous phase is observed in the equilibrium state. Furthermore, we investigate the size dependence of structural changes in the polydisperse system. In consequence, we observe less of a dependence on the phase changes in our oscillatory shear case.