This paper focuses on the application of digital gradient sensing (DGS) method to the determination of stress intensity factors (SIFs) for cracks in polymethyl methacrylate (PMMA). First，the full-field displacement fields of the test sample are recordedusing digital image correlation (DIC), and then the angular deflections of light rays propagating through the material are obtained based on the elasto-optical effect exhibited by transparent materials subjected to non-uniform state of stress. Under plane stress conditions, the deflections of light rays can be related to two orthogonal in-plane stress gradients. In this way, the stress gradients near the crack on PMMA plates are quantified, and then cracktip SIFs are extracted from DGS Measurements using the method of nonlinear least-squares proposed. With the obtained SIFs, crack tip location is determined simultaneously. For validation, the SIF determined is compared with theoretical results, confirming the effectiveness and accuracy of the proposed technique. Therefore, it DGS method is a practical and effective tool for SIF measurement.