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As the highest power proton linear accelerator in continuous wave mode, China initiative Accelerator Driven sub-critical System linear accelerator will deliver 2.5 MW proton beam to the target inside the reactor. Vacuum transition from 5×10^{4}    Pa to several 10^{-6}  Pa is required at the accelerator to target section. A vacuum differential system located at the transfer line is designed to meet the requirement of the windowless target and upstream collimators are considered for power loss control in this paper. A vacuum test based on vacuum transition verifies that about 10 orders of magnitude vacuum transition could be achieved as required, and gas scattering simulation shows that the power loss and beam size increase are at an acceptable level before the last magnet.