The Danish government set an ambitious goal to achieve a fully renewable-based energy system by 2050. In this context, the integrated electricity and heating system is undergoing rapid development in Denmark as a promising way to accommodate the ever-growing renewable energy sources (RESs). The electric heat pumps (HPs), coupled with the power and heat sectors, can propagate the flexibility on the heat consumer side to the power system operations, playing an important role of improving system flexibility and balancing the variability of the RES. In this paper, the current development situation of HPs in Denmark is analyzed, including both the large-scale HPs in the district heating system and individual HPs on the residential side. The possibility of using HPs to provide frequency control ancillary service (FCAS) is analyzed according to the market and technical requirements of the FCAS in the Danish transmission system and experimental results of representative demonstration projects of HPs. A comprehensive analysis of the advantages, barriers, future prospects, and challenges for using HPs to provide the FCAS are carried out from the perspectives of different entities.