To change the practice by the dispatch departments of taking over the peak-regulating ancillary services of power plants for free use，procedures have been promulgated by the State Electricity Regulatory Commission for dividing ancillary services management of power plants. But these procedures are still based on the qualitative method in regard to the division of paid and free peak-regulations， thus lacking theoretical support. An analysis is made of the impact of low-load peak-regulation on the units operation in regard to security and economy. The no-oil-consumption stable combustion limit and maximum load-reducing rate are chosen as indicators. The method and criteria for dividing paid peak-regulation and free peak-regulation based on convex hull barycenter-seeking are proposed. This dividing method uses the barycenter of the units' two-dimensional feature points distribution as the general peak-regulating indicator. An example is given to analyze and verify the feasibility and practicability of the dividing method.