Abstract: For studying crack propagation process, four-point shearing specimens, with the maximum size of 2500mm×600mm×200mm, are utilized to systematically investigate the crack initiation, stable propagation and unstable failure of concrete by means of photoelastic coatings. Using camera, the complete crack propagation process of concrete that appeared as interference fringes on the photoelastic coatings was recorded and the whole process of stable propagation and unstable development of the main crack was recorded. Based on the colored strain sequence, the strain distribution around the crack tip was measured and the curve of P-∆a was also obtained. In the presence of the crack extended length of photoelastic coatings, the finite element method was applied to simulate the process of crack propagation and count the variation of stress intensity factor at the tip of crack during crack extension. The results show that there is an obviously crack stable propagation before an unstable propagation, and that it can be recorded using photoelastic coatings. Therefore, the failure criterion, taking no account of ∆a, is conservative. It can be also concluded that fracture mode has been turned into I mode from Ⅰ-Ⅱ mixed mode after crack initiation, although the loading condition is Ⅰ-Ⅱ mixed mode.