The invention particularly relates to a device for preparing sodium oxalate from carbon dioxide and water by electrolysis and a preparation method. The device for preparing the sodium oxalate from thecarbon dioxide and the water by electrolysis comprises an anode circulating loop, a cathode circulating loop and a crystallization evaporation system; the preparation method comprises an electrolysisstep and an evaporation crystallization step; catalytic electrolysis is performed on a sodium salt solution by an electrolysis system by taking the carbon dioxide as a raw material to produce a sodium oxalate solution; when reaching 5 percent to 10 percent in concentration, the sodium oxalate solution is introduced into the crystallization evaporation system for crystallization evaporation; afterbeing introduced into the crystallization evaporation system, the sodium oxalate solution undergoes an acidizing process of an acidizing tank, a process of removing carbon dioxide gas by a degassingtank, a process of performing crystallization evaporation on the sodium oxalate solution by a first-stage evaporator and a second-stage evaporator, a process of separating sodium oxalate saturated solution from sodium oxalate crystals by a centrifugal machine and a process of drying the separated sodium oxalate crystals by a drying machine; and finally, sodium oxalate crystal particles are obtained.