The invention discloses an automatic cutting-off device and a cutting-off process for an aluminum flat tube. The automatic cutting-off device comprises a cutting-off device body, wherein the cutting-off device body comprises a necking cutting device and a notch pulling-off device; the necking cutting device comprises a first base, a assembly fixing plate, a driving mechanism, a central connectinggear, a necking knife assembly and a bisection knife assembly, wherein the assembly fixing plate is fixed on the first base, the central connecting gear, the necking knife assembly, the bisection knife assembly and the driving mechanism are installed on the assembly fixing plate; and the notch pulling-off device comprises a second base, a locking device and a pulling-off device body, wherein the locking device is fixedly installed at the right end of the second base, a transverse sliding rail is arranged at the left end of the second base, a transverse sliding groove is formed in the bottom part of the pulling-off device, and the second base is in sliding connection with the pulling-off device. The cutting-off process determined by the device comprises the following steps of conveying, necking and cutting, conveying to the notch pulling-off device, pulling-off, conveying and collecting. The cutting-off device and the cutting-off process have the advantages that structure is simple, operation is stable, operation is convenient, and size deviation caused by shaking of the device is overcome, and the precision is high.