Monthly maintenance schedule has an important guiding significance to day ahead maintenance arrangement. However, the current management of monthly maintenance schedule is short of quantitative analysis methods. Consequently it can’t effectively take the security risk that may exist under monthly maintenance schedule. This paper presents a static security checking method for monthly maintenance schedule. First, established the monthly power grid network model based on the realtime model and device state changes, including the commissioning and decommissioning devices and maintenance devices. Next, the month-contract energy was decomposed to every day and every hour, then combined with monthly load forecast and maintenance schedule, the power system typical operation modes were generated automatically. Finally, quantitative assessment of monthly maintenance schedule was completed by power flow calculation, security analysis and evaluation indexes, including load supply adequacy and equivalent generation capacity loss. This method is instructive for monthly maintenance arrangement and its feasibility is proved by practical application.