The invention discloses a device for defending the attack of an avalanche transition region. The device comprises a clock fan-out device, a delay module, a multiplexer device, a random source device and a gate signal generating circuit device; the clock fan-out device is used for dividing a departure input signal passing by a detector into two paths of independent clock signals T1 and T2; an output end OUT of a multiplexer is connected with the gate signal generating circuit, the minimum delay between a trigger input signal and a gating output signal is t, and the gating output signal output by the gate signal generating circuit generates random jitter at a time t or t+ delta t and is output by an output joint J2 to APD. The invention also discloses a method for defending the attack of theavalanche transition region. By adopting the device and the method disclosed by the invention, the attack of the avalanche transition region is detected while ensuring the counting stability of the detector.