The day-ahead generation scheduling module of North China Power Grid security constrained convention dispatch auxiliary decision-making system achieves two major functionalities， unit commitment and security constrained economic dispatch. The optimized result of unit dispatch schedule satisfies the security constraints and auxiliary service requirements. Based on the same optimisation model， different dispatch modes can be realized by different input data， such as the energy-saving generation dispatch， generation scheduling dispatch， cost dispatch， and market competition/clearing dispatch. The violation and violation penalty weights are introduced to guarantee convergence and satisfy the actual engineering application requirements. The computation time is reduced by the iteration of the simultaneous feasibility test and optimisation computation. The optimisation model of day-ahead generation scheduling belongs to mixed-integer programming and the branch-and-bound algorithm is adopted to solve it. To accurately check the day-ahead dispatch results， the study-state whole-network multi-timepoint security check function is developed to ensure the scheduling result meets the power system security requirements.