The measurement aperture is not matched when the wavefront sensor detects the free-form lens, and the measurement process and the data processing are cumbersome in the splicing detection method. To solve the problems above, a method is proposed for measuring the free-form surface lens. The wavefront aberration of a free-form surface lens is detected after expanding the diameter of a single measuring beam, and then the shrinked beam with the wavefront information of the beam expanding measurement region is received by the wavefront sensor. The sub-aperture wavefront aberrations of four diameters expanded by 10 times were obtained by designing the optical path measurement, and the wavefront reconstruction and splicing of the beam expansion sub-aperture were completed. In order to verify the reliability of the experiment, the measured beam aperture is reconstructed and compared with the subaperture peak and valley values generated by the Hartmann sensor, and the flatness of the beam aperture of the beam expander is observed. The experimental results show that the method can effectively increase the measurement range of the small-bore wavefront sensor and improve the detection efficiency. It can be used for the detection of wavefront aberration of large-diameter lenses.