Abstract: According to the features of ADA concurrent programs, a new theory is introduced, which can be used to detect most deadlocks in an ADA program. A deadlock detecting tool named DDTT（dynamic deadlock testing tool） relying on this theory is designed. This detecting tool can be used to detect communicating deadlocks in a multi task ADA program. The theory and detecting rules used by DDTT are described in detail. Finally, the working principle of DDTT is introduced.