In the debonding process of multi-filamentary unidirectional fiber-reinforced composites, the broken components (fiber and matrix) and debonded regions interact with each other. In this work, a new two-dimensional shear lag simulation method has been developed to investigate such interactions under the influences of the residual stresses and frictional shear stress at the debonded interface and it has been applied to obtain the outlines of the debonding process. It is shown that (1) the debonding is hastened by the interactions; (2) the existence of the frictional shear stress at debonded interface reduces the growth rate of the debonding; and (3) the existence of tensile and compressive residual stresses in the longitudinal direction in the matrix and fiber, respectively, acts to hasten the growth of the debonding when the broken component is matrix, while it acts to retard when the broken component is fiber.