As a widely used reactor, the circulating fluidized bed (CFB) has its solids circulation rate range from 5 to 1400 kg/m(2)s in the industry. However, almost all of the studies in the literature have their solids circulation rate below 300 kg/m(2)s. With the systematical optimization, the high solids circulation rate, up to 700 kg/m(2)s, has been achieved and instantaneous solids holdup has also been measured using an optical fiber probe. According to probe signals, two structures have been identified with solids holdup characteristics: "dilute structure" featured with dispersed aggregations and "dense structure" featured with continuous aggregations. Then, by analyzing the signal details, the instantaneous flow structures were further explored with the phase identification and characterization. Collectively, this work significantly contributes to the understanding of the flow information in high-density conditions and could dramatically benefit the development and validation of numerical models.