The invention provides a low permeability coal bed liquid nitrogen freeze thawing cracking permeability increasing method. The low permeability coal bed liquid nitrogen freeze thawing cracking permeability increasing method comprises the following steps that a, a freeze thawing drill hole and a monitoring drill hole are drilled; b, a first pipeline is arranged, one end of the first pipeline extends into the freeze thawing drill hole, the other end of the first pipeline is connected with a nitrogen generation separating device, and the nitrogen generation separating device is connected with a liquid nitrogen injection pump; c, a temperature monitoring device is installed in the monitoring drill hole, after the temperature monitored by the temperature monitoring device is stable, the temperature at the moment is determined to service as the initial temperature T0; d, the liquid nitrogen injection pump is started, the nitrogen generation separating device is driven to inject liquid nitrogen into the freeze thawing drill hole, and the temperature of the freeze thawing drill hole is monitored in real time; e, when the temperature T in the monitored drill hole is reduced by 50%-80% compared with the T0, the liquid nitrogen is stopped from being injected, and the temperature in the drill hole is monitored continuously; f, when the temperature T in the monitored drill hole is increased to the condition when |T-T0| / T0<5%, the step d and the step e are repeated; g, the step f is repeated for 3-5 times. By means of the low permeability coal bed liquid nitrogen freeze thawing cracking permeability increasing method, the pore structure of an original coal body is modified, the problem that the permeability effect of a low permeability coal bed is poor, and the permeability increasing efficiency of the low permeability coal bed is effectively improved.