We simulated the micromechanical damage and strength of discontinuous carbon fiber-reinforced thermoplastic composites with a Shear-Lag Model (SLM) and compared the SLM with a Continuum Damage Mechanics (CDM) with finite element model in order to clarify the potential of SLM. As a result of tensile failure simulations, the ultimate tensile strengths in the two dimensional models (CDM and 2D-SLM) are less than that in 3D-SLM because of unphysical high stress concentrations when the fibers are long. Therefore for long fibers, 3D-SLM is the most appropriate method. In order to investigate from which fiber length 3D-SLM can be used effectively, the transition length for failure mode is examined. In this study, the strain at the peak stress is used to determine the transition length of failure mode quantitatively. Finally, strengths with 2D-and 3D-SLM are compared with experiments for long discontinuous fiber-reinforced thermoplastic composites. The strength obtained by 3D-SLM is higher than experimental results, and 2D-SLM predicts strengths much less than experiments; nevertheless, the simulated results lead to an ideal strength. Therefore, 3D-SLM is the most appropriate of the models discussed in this study for predicting the strength of long discontinuous fiber-reinforced thermoplastic composites.