Abstract: Time domain reflection is a method for detecting cable's fault positions and reasons by analyzing the relationship between the transmitted pulse and the reflected pulse. Aimed at the blind spot in the domain reflection with narrow transmitted pulse, a method using wide differential transmitted pulse is proposed. The cable length and the fault reasons can be located by the method and the blind spot can be eliminated. The hardware structure of network cable tester designed by the method is presented and the key parts pulse generator and reflected pulse analyzer are analyzed in detail. At the end, the experiment data obtained by network cable tester are analyzed, the results show that the method is reliable.