As a rapidly developing technique of photomechanics, the digital speckle correlation method (DSCM) has attracted attention for its advantages of noncontact, real time, whole field and wide measuring range, etc. Raising the speed of convergence is a developing direction of the method for its great calculation volume. We present an improved DSCM by introducing an optimization theory. It can reduce the computational workload and speed the convergence velocity. This method is used to measure the displacement fields and strain fields in the crack tip region of thin foil, and the stress intensity factor is obtained by the experimental method. Applications show that the DSCM is an effective experimental technique for investigating the mechanical behavior of thin foil materials.