The invention discloses a device and a method for determining dissolved nitrous oxide in water and relates to detection of dissolved nitrous oxide in water. The device comprises a nitrogen cylinder, a first pressure reducing valve, a second pressure reducing valve, a mass flow controller, a flow meter, a six-way valve, a first three-way valve, a second three-way valve, a Nafion tube, a semiconductor cooler, a scavenging conduit, a trapping tube, a Dwarf can, an electric heating tube, a twelve-way valve, a Kelun pump, a sample bottle, a gas chromatograph and a computer. The method comprises a sample introduction stage, a scavenging stage, a trapping stage, a desorption stage and a separation and detection stage. Short time is consumed, the sensitivity is high, the automation degree is high, the operation is simple, the temperature can be subzero 100 DEG C or lower with adoption of the Dwarf can for accommodating liquid nitrogen to serve as a low-temperature source of the trapping tube, and nitrous oxide gas can be trapped on a molecular sieve completely. The electric heating tube can be heated to the temperature of 100 DEG C or higher within 0.5 min, so that the nitrous oxide gas can be desorbed rapidly to enter the gas chromatograph. The concentration of the nitrous oxide gas can be determined accurately by means of the Kelun pump.