The invention relates to a composite oxide catalyst for low and medium-temperature decomposition of N2O and a preparation method thereof. A nickel-lanthanum composite metal oxide is used as a major active component, one or more of alkaline-earth metal, transition metal and rare-earth metal element are used as promoters, and one of molecular sieves, honeycomb ceramics and active carbon particles is used as a carrier; and based on the carrier mass, the mass load percentage of the active component is 10-25%, the mass load percentage of the promoter M oxide is 0-10%, and the molar ratio of Ni to La is 1:(0.1-1). The pretreated carrier is equivalently dipped in a precursor composite solution of the active component, and a catalyst is prepared by drying and roasting. The catalyst provided by the invention has higher low-temperature catalytic activity, namely that N2O can be completely catalytically decomposed at 400 DEG C while the existing catalyst can completely decompose N2O generally at a temperature over 500 DEG C; and the catalyst provided by the invention has good stability, high mechanical strength and long service life. Under conditions of normal pressure and air speed (GHSV) of 2,400h<-1>, the catalyst provided by the invention can completely catalytically decompose 5,000ppm N2O at 400-470 DEG C.