The influence of interface performance on the macroscopic tensile properties for the random short spruce fibers reinforced polypropylene (PP) composite materials was investigated. The mechanical behavior of the imperfect interface between spruce fiber and PP matrix is described by the bilinear cohesive zone model (CZM), while a two-dimensional finite element model of the representative volume element (RVE) with CZM for the material was developed in terms of the volume content, aspect ratio (AR) and random anisotropic elastic of random distribution short spruce fiber, as well as the influence of elastic plastic PP matrix. Experimental tensile stress strain curves for the composites with different fiber volume contents were simulated. The results show that there exists a common trend of monotone increasing for the curves of imperfect interfacial stiffness versus the effective modulus, namely E-K curves. The E-K curves for the composites with different volume fraction of fibers converged to a unique critical point (CP). In the range of higher interface stiffness the effective modulus of composites increase with the increase of fiber volume content, in the range of lower interface stiffness that is the opposite. For three spruce/PP composites with different fiber contents of 10%, 20% and 49% (in volume fraction), their imperfect interfacial stiffness could be estimated by their E-K curves and the measured macroscopic effective elastic modulus through experiment. The displacement corresponding to the initial separation and that to the entire separation of the interface could also be determined by the simulating tensile experimental stress strain curve of spruce/PP. Therefore, the results of numerical analysis base on the imperfect interfacial stiffness can be used to explain and further understand the influence of random short fiber volume content on the effective modulus of spruce/PP composites.