•The pressure drop of unitary and binary pebble beds are studied.•In unitary beds, the "viscous" term coefficient varies with the void fraction.•In binary pebble beds, the "inertial" term coefficient depends on diameter ratio.•In binary pebble beds, the "viscous" term coefficient depends on bed homogeneity.•Increased diameter and volume ratio in binary beds lower tortuosity coefficient n. This paper presents a study on the characteristics of purge gas pressure drop in unitary and binary pebble beds, considering different void fractions and diameter ratios. To model tritium purge gas in the breeder zones of blankets, a helium loop is constructed to provide a flow at 0.1–2.0 MPa with a maximum flow rate of 80 Nm3/h. The experimental results show that the pressure drop ∆P increases linearly with superficial velocity, temperature, and gas pressure, and is inversely related to particle diameter and void fraction. In unitary pebble beds, the coefficient for the "inertial" term remains constant and is unaffected by the void fraction, while the "viscous" term coefficient varies with the void fraction. In binary pebble beds, the pressure drop is influenced by the diameter ratio and volume fraction of large particles. The "inertial" term coefficient depends solely on the diameter ratio and increases with it, while the "viscous" term coefficient is influenced by the tortuosity packing coefficient which decreases as the particle diameter ratio and volume fraction of larger particles increase. These findings of pressure drop are significant for improving tritium extraction performance during the design process of the blanket system.