It is a tough question to obtain an unwrapping phase when the under-sampling happens. Although the least-square algorithm based on lateral shearing interferometry (LSBLS) has good anti-sub sampling ability, a satisfactory result can not be obtained when the under-sampling is serious. To solve this problem, a corresponding analysis is presented and the LSBLS algorithm is improved by introducing a second shearing. The validity of this improved algorithm is showed in one simulated result and one experimental result. It is promised, by the algorithm, the validity of unwraped phase, when the absolute value of discrete secondary derivative of wrapped phase is less than π.