The invention discloses a device for relative time synchronization based on ultrashort pulses. The device comprises a first time signal input interface, a second time signal input interface, a first femtosecond laser, a second femtosecond laser, a time delay line, an ultrashort pulse nonlinear frequency doubling medium, a photoelectric probe and a time delay line feedback control circuit. The invention further discloses a method for relative time synchronization based on the ultrashort pulses. According to the technical scheme disclosed by the invention, time delay caused by time jitters of two paths of time pulse sequences can be resisted, high-precision relative time synchronization can be realized, and the synchronization precision is less than 50ps.