Twinning induced plasticity (TWIP) steel exhibits high strength and exceptional plasticity due to the formation of extensive twin under mechanical load and its ultimate tensile strength and elongation to failure-ductility-value can be as high as 50000 MPa%. Therefore, the TWIP steel can still maintain high energy absorption performance and impact resistance when its thickness is reducing to the half. The high work hardening plays a dominant role during deformation, resulting in excellent mechanical properties. The deformation mechanisms, responsible for this high work hardening, are related to strain-induced microstructural changes, which are dominated by slip and twinning. Different deformation mechanisms, which can be activated at different stages of deformation, will strongly influence stress-strain response and microstructure evolution. In order to understand the effects of slip and twinning during plastic deformation process, it is important to explore the microstructure evolution of those two deformation mechanisms and their influences on macroscopic deformation during this process. In this work, a crystal plasticity constitutive model of TWIP steel coupling slip and twinning was developed based on the crystal plasticity theory. In this model, the volume fraction of twin and its saturation value were introduced in order to consider the effect of twinning on hardening and slip, respectively. The constitutive model was implemented and programed based on the ABAQUS/UMAT platform. It was applied to simulate the plastic deformation process of single crystal for typical orientation microstructures under simply loading condition. The microscopic mechanism of plastic deformation of single crystals with different orientations was analyzed, and then the influence of slip-twinning system startup states on macroscopic plastic deformation was investigated. The saltation of stress for brass and S orientations was paid attention especially, the stress steep fall for copper single crystal was also reproduced during tensile tests. The results show that when the volume fraction of twin is small, its effect on strain hardening should be ignored; however, its impact becomes gradually obvious with the increase of volume fraction of twin; when the volume fraction of twin reaches saturation value, twinning increment is zero, the slip directions in crystal must change, another slip system will be activated as a result of stress dropping suddenly.