The invention provides a method and system for determining minimum annual utilization hours of a small-capacity set in a power grid. A typical day of a power grid load is determined according to an annual load curve of the power grid; on the basis of the load data of each typical day, a set maintenance plan and a set heat-supply steam-supply demand, grid safety stability checking and startup modeoptimization are carried out on the typical day to obtain an optimal start-up mode for start-up minimization of small and medium-sized sets of each typical day; the determined typical optimal start-upmodes are extended to be whole-year divided-period start-up mode combination according to characteristics of the annual load curve of the grid; and lowest annual utilization hours of the small and medium-sized sets are calculated according to the annual load curve and the whole-year divided-period start-up mode. According to the method and system provided by the invention, with full considerationof constraint conditions and influence factors like safe and stable operation of the power grid, peaking degrees in summer and winter, set maintenance and heat-supply steam-supply demands, the calculation accuracy is improved and the high rationality of calculation results is ensured.