Based on the basic idea of the Kachanov method, the stress intensity factors for the interaction of isometric collinear cracks are computed. Moreover, according to the Mises failure criterion and the D-P failure criterion, the expression of the plastic zone radius at the crack tip and the expansion ratio of plastic zone radius under the crack interaction are derived based on the small-area yield condition under the uniaxial tension. The radius enlargement of the plastic zone is the square of the ratio of the stress intensity factor under the interaction of two cracks to the stress intensity factor of a single crack. And, the influence of crack spacing on the plastic zone expansion is also analyzed. The results show that, for a same inclining angle of crack, the plastic zone radius of crack tip increases with the decrease of crack spacing, and its increasing rate increase as well. For different crack angles, the contact conditions at the plastic zone of crack tip are also different. Therefore, the combination conditions of double cracks need to be determined according to the inclining angle of crack.