The present invention discloses a preparation method for a catalyst for synthesizing oxalic ester by gas-phase. The catalyst takes alpha-alumina as a carrier, palladium as an active component and 2 MOxes as an additive, M is magnesium, titanium, zirconium, vanadium, manganese, iron, nickel, copper, zinc, molybdenum or tungsten, and components of the catalyst (calculated by carrier mass) are: 0.01 to 0.75percent of the palladium and 0.1 to 20 percent of MOxes. The preparation method comprises the following steps that: firstly, an additive metal salt solution is used to impregnate the carrier, and a palladium salt solution is used to impregnate the carrier to obtain the catalyst after the carrier is dried and roasted. Before the use, pure hydrogen or H2-N2 mixed gas is activated by the catalyst at a temperature of between 150 and 450 DEG C. The catalyst can be used for synthesizing oxalic ester by the carbonylation of CO and nitrous acid ester, the using reaction temperature of the catalyst is between 70 and 150 DEG C, and the reaction space velocity is between 500 and 9,000h<-1>. The catalyst has higher reaction activity and selectivity; and the catalyst has low cost.