In this paper, a two-dimensional microstructure-based finite element modeling method is adopted to investigate the effects of material parameters of the constituent phases on the macroscopic tensile behavior of Q&P steel and to perform a computational material design approach for performance improvement. For this purpose, a model Q&P steel is first produced and various experiments are then performed to characterize the model steel. Actual microstructure-based model is generated based on the information from EBSD, SEM and nano-indentation test, and the material properties for the constituent phases in the model are determined based on the initial constituent properties from HEXRD test and the subsequent calibration of model predictions to tensile test results. The influence of various material parameters of the constituents on the macroscopic behavior is then investigated. Based on the observation of the respective influence of constituent material parameters, a new set of material parameters is devised, which results in better ductility performance. The results indicate that various material parameters may need to be concurrently adjusted in a cohesive way in order to improve the performance of Q&P steels. In summary, higher austenite stability, less strength difference between the phases, and higher hardening exponents of the phases are generally beneficial for performance improvement. The information from this study can be used to devise new Q&P heat-treating parameters to produce Q&P steels with improved performance.