In this paper, the principle of coherent gradient sensing (CGS) technology is briefly introduced. Fracture behaviour of the PMMA specimens with single or double cracks under three-point-bending are studied using the CGS optical method. A series of CGS fringe patterns under different loading are abtained. With these fringes, stress intensity factors are caculated. The influences of the different spacing on stress intensity factors are discussed. These results will be useful for researching on strength of PMMA materials.