The invention provides a device for detecting abnormal detachment of mobile terminals with session right and method thereof, which is applied to a cluster system comprising PDS, a plurality of BSC anda plurality of mobile terminals, wherein, the method includes: step one, after a session right request of the mobile terminal in the cluster system is received, PDS sends terminal session right authorization messages to partial or whole BSC of the cluster system; step two, after mobile terminal session right authorization messages are received, partial or whole BSC returns status notification messages to PDS, if abnormal detachment occurs, does not return status notification messages; step three, if any status notification message is received in a certain collecting cycle, PDS releases session right of mobile terminals with session right, if at least one status notification message is received, maintains status of mobile terminals with session right, realizes hard handoff between BSC, also can track status of mobile terminals with session right.