Abstract: The sampling rate of an underwater acoustical synchronous positioning system for the track of an underwater target will be decreasing during the process of positioning while the target moves away,resulting in the reduction of raw data and insufficient use of the processing ability of the positioning system.For a long time,this problem remains unsolved,and it is even pushed forward recently because of the rapid development of modern electronic techniques.Based on the thorough study and investigation of the problem,we developed a new synchronous positioning technique called "cross sync-period" underwater acoustical positioning.It can increase the sampling rate of an underwater acoustical positioning system for the track of an underwater target at long range significantly.Besides,a new algorithm specially designed for the detection of the propagation time delay of positioning signals,called "moduloalgorithm",was also developed,which makes the implementation of the"cross sync-period "underwater acoustical positioning technique easier and more efficient.These techniques have been successfully applied in a real positioning system.The system can position 5 underwater targets at the maximum range of 6 Km simultaneously without any ambiguity of target distances with the working period of 0.4second.Performing the" cross sync-period"underwater acoustical positioning technique by the system in lake and sea-run tests,we obtained satisfactory results