With UHVDC construction and renewable energy development, DC blocking faults bring challenge to fast frequency regulation of power system. To improve frequency stability of power system with energy storage technology, a demand assessment method is proposed for energy storage to participate in fast frequency regulation of power system. Firstly, bottleneck evaluation index and bottleneck scenario analysis method of fast frequency regulation are proposed to acquire scenario basis for capacity configuration of energy storage. Then an optimal configuration method on capacity of energy storage is proposed based on bottleneck boundary scenario, which eliminates bottleneck scenarios with the minimum cost of capacity configuration of energy storage. Finally, the effectiveness of the proposed method is verified via case studies based on actual data of Jiangsu grid.