The invention discloses an efficient and isothermal preparation method for a catalyst for preparing methane from coal-based syngas. The method is used for solving the problem that the existing methane catalysts cannot have both stability and production cost. The technical scheme is as follows: the method comprises the following steps: (1) dissolving an alkali metal or alkaline-earth metal salt, which is easily soluble in water, in water, impregnating a support with the assistance of ultrasonic, and then, carrying out step-by-step drying and roasting, so as to obtain a modified support, i.e., M1/Support; (2) dissolving nickel nitrate and a rare-earth metal salt in an aqueous solution together, adding the support, i.e., M1/Support into the obtained solution to obtain a suspension, adding an alkaline solution into the suspension under the conditions of stirring and ultrasonic assistance to control the pH value of the suspension, and carrying out precipitation; and (3) subjecting the precipitated solution to filtering, and carrying out washing, step-by-step drying and roasting, thereby obtaining the catalyst, i.e., NiM2/M1/Support, wherein the catalyst contains 5% to 35% (by mass) of active ingredient NiO, 0.5% to 10% (by mass) of alkali metal or alkaline-earth metal oxide, 0.5% to 10% (by mass) of rare-earth metal oxide and the balance of support. According to the method, the process is simple, the production cost is low, and advantages of two traditional catalyst preparation methods, i.e., a coprecipitation method and an impregnation method are combined.