The invention provides a nanosecond pulse width measurement device based on a FPGA and a method. When pulse width measurement is carried out, a double-channel time interval measurement mode is employed, one channel utilizes a rising edge of an input pulse to carry out synchronization for a gate, the other channel utilizes a falling edge of the input pulse to carry out synchronization for a gate, the synchronization gates are both inputted to the FPGA, the FPGA is used for monitoring the two paths of synchronization gates, when signals of the two paths of synchronization gates are monitored to be high, after delay for a time segment, the synchronization gates of the two channels are pulled down, so the new synchronization measurement gates of the two channels are acquired, time counting and TDC interpolation compensation for the new synchronization gates inputted to the corresponding channels are carried out by each corresponding channel, and the width data of the synchronization gates is acquired; subtraction operation for the two paths of synchronization gates is carried out to acquire the pulse width data of the input pulse. The method is advantaged in that smallest pulse width which can be measured is lower than 1 nanosecond.