The explosion process of the flake aluminum powder–air two-phase flow is experimentally studied in a large-scale long straight horizontal tube with a length of 32.4 m and an inner diameter of 0.199 m. The deflagration-to-detonation transition (DDT) of the aluminum powder–air mixture is analyzed after being ignited by a 40-J electric spark, and the DDT of the mixture at different mass concentrations is compared. The results show that self-sustained detonation can be achieved in the range of 286–532 g/m 3  of the flake aluminum powder concentration, and the DDT process of the aluminum powder–air mixture at the concentration of aluminum particles 409 g/m 3  (optimal concentration) is analyzed in detail. The detonation velocity and detonation pressure at the optimal concentration are 1690 m/s and 58 bar, respectively. During the self-sustained detonation stage, the detonation overpressure of the multiphase fuel–air mixture exhibits a typical constant oscillation characteristic, while the detonation velocity remains stable. In addition, a double-headed mode helical detonation phenomenon is observed in the detonation wave front of the aluminum powder–air mixture. The structure of the detonation wave, the flow field parameters, and the interaction between the shock wave and the three-wave point trajectory are analyzed. The detonation cell size at the optimal concentration is approximately 486 mm.