Optical bistability (OB) and optical multistability (OM) due to wide applications in all-optical switching and transistors is studied in this paper. Here, we study the OB and OM properties of incident light in a defect slab doped by a GaAs quantum well (QW) nanostructure. It is shown that OB and OM features can be manipulated by spin coherence created by circular polarized laser fields in GaAs QWs. The impacts of laser field features, such as intensity, frequency detuning, and relative phase, on OB and OM are simulated. Moreover, the dependence of OB and OM features of a probe light on the thickness of the slab, then, are analyzed. It is found that the thickness of the slab can provide a new way to optimize the intensity threshold of OB and OM. We hope that our proposed model may be useful for developing all-optical devices on nanoscales.