The invention discloses a liquid-nitrogen-assisted gas cylinder static evaporation rate detection BOG recycling device and method. The device comprises a liquid nitrogen assisting device, a first LNG storage tank, a second LNG storage tank, a third LNG storage tank and a control box. The method comprises the steps that BOG discharged in a gas cylinder natural emptying stage is recycled through liquid nitrogen assistance, the second LNG storage tank and the third LNG storage tank are connected in parallel through a pipeline and a valve and then connected with the liquid nitrogen assisting device and the first LNG storage tank in series through a pipeline and a valve, and discharged BOG is alternately recycled in normal-pressure gas cylinder 24-h standing and static evaporation rate detection stages. BOG is recycled in a gas cylinder static evaporation rate detection process, the problem that a detection effect is affected due to the fact that the LNG storage tanks reach saturation in the absorption process is effectively prevented, meanwhile, the problem that pressure is too high due to LNG volatilization in the LNG storage tanks is effectively avoided, and the device is higher in practicability.