The time-interval based optimization algorithms are used to solve the monthly unit commitment and security assessment. Aimed at two modes of power market and energy efficiency dispatch， the corresponding mathematic optimization models are proposed. Lagrange relaxation method is used to optimize the monthly unit commitment over 720 time intervals， and the non-linear interior point optimal power flow（OPF） is used in the security assessment and optimal adjustment. Taking the extended IEEE RTS system and the actual East China power system as examples， the large-scale system analysis is carried out. The results show the efficiency of proposed algorithms for monthly generation schedule， security assessment and optimal adjustment.