The field-dependent Young's modulus shows a promising application in the design and miniaturization of phononic crystals, tunable mechanical resonators, interdigital transducers, etc. With the multi-field bulge-test instrument developed by our group, the electric field-tunable elastic modulus of ferroelectric films has been studied experimentally. A butterfly change in Young's modulus of PZT film under biaxial tensile stress state with electric field has been discovered for the first time. Based on phase field model, an electro-mechanical coupling model is constructed and a case of PZT ferroelectric film subjected to a vertical electric field and horizontal tensile strains is simulated. The numerical results show that the change in Young's modulus is proportional to the variation of volume fraction of 90o domain switching under a pure tensile strain. It is the constraint of 90o domain switching by electric field that contributes to the butterfly change in elastic modulus.