As an intense neutron sources, the high power spallation targets have important application value in the nuclear transmutation, nuclear synthesis, material research and so on. Dense granular-flow targets have the advantages of both solid and liquid targets, and make them become one of the most candidate promising solutions for the future high power targets. Here, the proposal and development of the concept of dense particle flow targets were reviewed. Then, several key issues related to flowability in the design of the targets were reviewed, including the establishment and optimization of coupling algorithms for the beam target, the exploration of the prediction algorithms for the particle material and flow characteristics, and the optimization of the overall flow rate and local flow field. Finally, the future direction of the dense granular-flow in high power target research was prospected.