The purpose of this paper is to investigate the mixture formation and optimize the operating conditions under cold start in a stoichiometric (λ=1) GDI engine with wall-guided piston using a 3D commercial code, STAR-CD . For GDI engine under cold start, it can be difficult to carry out the optimization of operating conditions by engine test alone without the understanding of mixture formation inside the combustion chamber. In this study, three cold start conditions of the catalyst heating mode with split injection, the cranking under freezing temperature and acceleration before engine warm-up which causes oil dilution were calculated. In particular, injection strategy for each cold start condition were optimized and compared to the engine test data.The previously validated spray models  were applied to the analysis of the spray formation and mixing process inside the combustion chamber. Also, Bai's droplet-wall interaction model and liquid film model considering the film stripping on the surface were used for better prediction of wall film behavior. This approach reasonably predicts the interaction of the injected spray and the in-cylinder flow, the mixture distribution around the spark plug, and liquid film on the wall inside the combustion chamber. The analysis results show that under cold start with relatively low engine speed, injection parameters such as pressure, split injection ratio and timing strongly affect tumble flow, mixture formation and wall wetting. The results for the optimized injection conditions are qualitatively in good agreements with experimental data in terms of combustion stability (RPM variation), HC emission, and oil dilution.