In the optical interferometry fields, the phase extracted by the arctangent function is a 2π-wrapped phase, it is necessary to carry out the phase unwrapping to obtain a correct continuous phase distribution. However the undersampled phase occurs due to too low sampling frequency and higher fringe density, thus the common unwrapping algorithms will fail. Aimed at the undersampled problem, a phase unwrapping algorithm based on lateral shearing and zooming is presented in the paper. The algorithm combines least square phase unwrapping based on second lateral shearing and bicubic interpolation to obtain larger anti-undersampled range. Taking peaks function as object, the anti-undersampled ranges are analyzed for different phase unwrapping algorithms. It can be shown that the presented algorithm retrieves the continuous phase of 2750 times the peaks function. The algorithm can provide a phase unwrapping solution for the serious undersampled phase, and the analyses of anti-undersampled capability for different phase unwrapping algorithms also are as a reference for future measurement.