Graphene oxide is attractive in flexible materials due to its biocompatibility. Herein, flexibility of graphene oxide under uniaxial tensile strain has been studied in term of the bending modulus. It is demonstrated that tensile strain plays an effective role in softening the bending modulus of graphene oxide. The strained graphene oxide can be even more flexible than graphene. The mechanism of strain softened bending modulus is discussed where weakened atomic bonding accounts directly for the enhanced flexibility. Particularly, an important factor for softened bending modulus is proposed, i.e. alignment of epoxide groups. The degradation of bending modulus is more prominent when the epoxides are aligned along the bending direction. This work not only proposes an effective method to enhance the flexibility of graphene oxide, but also unveils the mechanism of softened bending modulus, which could be useful in design of highly flexible materials.