The invention relates to the field of centrifugal enriched uranium, in particular to a centrifuge cascade centrifuge and a lower support system online replacement method thereof. The method comprises the following steps: (1) the section of the machine unloads, and pumpdown is performed; (2) the section of the machine is down from the cycle for shutdown; (3) the section of the machine undergoes vacuum breaking and air purge, and pumpdown is performed; (4) a damaged machine or bracket is replaced; (5) the lower support system is replaced; (6) section of the machine vacuum test and a freezing measurement are made; (7) section of the machine is up into the cycle, passivated, and input to a circuit. The method is capable of replacing the centrifuge and the lower support system during a cascade operation. The centrifuge cascade centrifuge and lower support system online replacement method thereof have the advantages of avoiding a full-cascade shutdown, having a good economic benefit, being reasonable, effective, operable and wide application prospect simultaneously.