The hydraulic jar in test string is used for jam release while DST is sticking. It bears drilling string's pulling power temporarily, and then, the power will be released after stretching drill pipe; an impact power will be affected on the stuck tool when drill pipe shrink. User can get multiple bumps by operating the jar time after time. Hydraulic jar's function of bearing pulling power in short time is supplied by the hydraulic time-delay system of the jar. When the metering cylinder move to bigger ID part of outer barrel to let special hydraulic oil flow from the out of metering cylinder, hydraulic jar's function of bearing power will lose and time-delay finish. The Delay Time to relieve bearing power function in short time will be changed with the power applied on the tool, that is to say, the power is smaller while the relieve time is loner.