The invention relates to a preparation method of a catalyst for synthesizing methacrylic acid. The preparation method comprises the following steps: preparing a heteropoly compound catalyst by adopting an ionic liquid and metal salt step-by-step precipitation method; performing a reaction on a molybdenum-containing compound, a vanadium-containing compound and phosphoric acid to obtain solution orslurry A; preparing imidazolium ionic liquid into solution B; preparing alkali metal nitrate, transition metal nitrate and lanthanide nitrate into mixed aqueous solution C; adding the B and the C into the A in a step-by-step precipitation and aging mode; and then obtaining the catalyst by concentrating, drying, forming and roasting. The catalyst prepared by the method has high activity and high selectivity for the reaction of preparing methacrylic acid by carrying out gas-phase oxidation on methylacrolein and has high mechanical strength and high thermochemical stability.