Abstract: A numerical compensation method is proposed to reduce nanosecond signal distortion caused by tens meters of cable. The method calibrates the target cable with a relatively short cable, performs Fourier transform on the output signals of the two cables, and then gets the original function of the long cable sensitivity and phase with frequency, by using the frequency characteristics of the signals. For this original function is full of noise, it needs to be the fitted with system model. And then the cable’s output signal can be compensated after the experiment signal being measured in frequency domain. The experiment shows that this method can improve the bandwidth of cable significantly. The 100 m-long SYV-50-7-2 cable whose bandwidth is just 40 MHz is improved to 1 GHz by compensation.