A method to measure the focal length of Microlens-Array (MLA) is proposed based on Hartmann-Shack wavefront measurement principle and the Focus Determining Technique by Image Definition(FDTID). The composition of the measurement system is introduced and the measuring principle is given. Firstly, the plane wavefront from a collimator is imaged nearby the focus of the MLA, and the spherical wavefront induced by the standard lens is also imaged on the focus. Then, the focal length of MLA is calculated by determining the excursion between plane wavefront and spherical wavefront measurements. Finally, an experiment based on the analysis of measurement uncertainty and the application of FDTID is preformed to validate the feasibility of the proposed method. The measuring results demonstrate that the focal length measurement precision is about 3% and several sub-lenses of the MLA can be determined at single shot. It concludes that this method has higher precision and efficiency in the focal length measurement of MLA as compared with traditional methods.