As there exists speckle noise in the phase of optical field obtained from holographic interferometry, it is necessary to reduce noise so that phase unwrapping can be successfully completed. Separating noise from the optical field through shearing is an effective method, but the study on influence of shearing quantity selection is still needed. Through theoretical analysis, simulation computation and experimental validation, it is proved that shearing quantity has influence on the linear correlation between the phase of ideal shearing optical field and the phase of shearing optical field containing noise, and the difficulty of noise reduction is changed. The phase linear correlation coefficient reaches a maximum through selecting appropriate shearing quantity, thus the best noise reduction result can be achieved.