In this work, we present an approach to use graphene oxide-silicon bi-layer flexible structure as stress-based humidity sensors. By the spin-coating method, graphene oxide thin films were deposited onto silicon microbridge as a humidity sensing layer. Upon expose to humid environment, graphene oxide thin films swells and leads to the bending of silicon membrane. Then, the full piezoresistive Wheatstone-bridge embedded in silicon microbridge was used to transform the deformation into a measurable output voltage. The humidity sensing properties of the bi-layer flexible structure, such as sensitivity, repeatability, humidity hysteresis, response and recovery, were investigated in the wide relative humidity range of 10–98%. The test results show that graphene oxide-silicon bi-layer flexible structure exhibits high humidity sensitivity, good repeatability, small humidity hysteresis and clear and fast response–recovery. Moreover, the dependence of the thin films thickness of graphene oxide on the response properties was also examined. At last, the humidity sensing mechanism of the proposed bi-layer structure was discussed in detail.