Standard experimental tests including photoelasticity in the evaluation of stress intensity factors in crack tip has been regarded widely. A least-squares method is used to determine the stress field parameters of interface crack in Aluminum/ epoxy bimaterial and its evolvement. Based on multi-parameter stress field equations and the least-squares principle, a set of over-determined nonlinear equations is established by fitting the isochromatic phase field obtained by digital phase-shifting photoelasticity in this paper. An iterative procedure based on Newton-Raphson method is utilized to estimate the unknown stress field parameters. Interface crack experiments reveal that the modulus of combined stress intensity factor increases with the applied loads and however its phase angle holds the line on the condition of same loading direction. On the other hand, the modulus of combined stress intensity factor increases with the loading direction from 30o to 75o under the same applied load, however, its absolute phase angle decreases and is independent of the loads.