The invention discloses a method and a device for water-sediment interface in-situ sampling and fixing. The device is supported by a self-gravity sediment column sampling support and comprises a sediment column sampling pipe and a liquid nitrogen refrigeration system. The sediment column sampling pipe comprises an inner sampling pipe and an outer liquid-nitrogen refrigeration chamber, and the liquid-nitrogen refrigeration chamber, a liquid-nitrogen input pipe, a unidirectional nitrogen discharge valve, and a self-pressurization-type liquid nitrogen tank form the liquid nitrogen refrigeration system. The sediment column sampling pipe is put into the self-gravity sediment column sampling support and is connected to the liquid nitrogen refrigeration system, when the water-sediment interface enters into the refrigeration chamber of the sampling pipe, the self-pressurization-type liquid nitrogen tank is started and fixes a sediment column, after the sediment column is fixed, the sampler is taken out of water surface, lake water is added into the refrigeration chamber, the solid sediment column slips out of the sampling pipe, and the sediment column is subjected to refrigeration preservation and transport. The method prevents deposit structure disturbance in sampling and transport adopting the conventional method and is convenient for secondary sampling analysis.