The effect of lighting conditions on digital speckle correlation method (DSCM) measurement accuracy is researched, and the optimal parameters and light condition for digital speckle correlation calculation are obtained. Through theoretical analysis of the model of DSCM, the relational expression of error about the frequency and average gray gradient is obtained. The optimal digital speckles size, fitting window size and calculation window size are got with computer simulation of digital speckle image. Leading into the technology of GSI in physical experiment, the accurate displacement is got and the error of different light conditions is analyzed, finding the connection about light with the frequency and average gray gradient through studying the digital speckle images. Experiments show that 8000 lx is the optimal light condition in DSCM, when average gray gradient is high and DSCM measurement is precision.