Abstract: It exists a contradictory relationship between ability of peak shaving and load rate. In order to improve the economy of electric power system operation and ability of peak shaving simultaneously, optimal load distribution model considering the primary frequency control ability（PFCA） was proposed. Furthermore, sine cosine algorithm（SCA） was introduced to solve it. Taking four units as simulation example to verify the model. The results show the SCA can achieve better performance than genetic algorithm（GA） in calculating optimal value. Compared with the automatic generation control（AGC）, the new method was more economical and meet the requirements of the PFCA. In addition, the relationship between optimal economic cost and primary frequency control capacity under different load rate was summarized by simulation. It has great guiding significance for load optimal distribution. Finally, load optimal distribution in low load rate was studied. The results show the units with better economy were selected preferentially to participate in deep peakregulating.