The invention relates to a method for synthesizing methyl isobutyl ketone from acetone through a one-step method. Acetone and hydrogen gas are taken as raw materials; a three-phase trickle bed reactor is adopted; the reaction conditions are that: the temperature is 120-160 DEG C, and the pressure is 2.5-4.0 MPa; the liquid air speed is 0.8-2.0 h<-1>; in a catalyst, gamma-Al2O3 is taken as a carrier after being modified by using a lanthanum salt for carrying metal palladium; the mass percentage content of lanthanum is 0.5-3.0 percent; and the mass percentage content of Pd is 0.03-0.10 percent. The preparation method comprises the following specific steps of: soaking a lanthanum component in an isometric way for gamma-Al2O3 to obtain modified gamma-Al2O3; and soaking metal palladium in an isometric way to obtain a catalyst, wherein the content of palladium serving as a catalyst is very low; the content of mesityl oxide serving as a byproduct is less than or equal to 0.07 percent; stability is kept for a long time; the acetone transformation rate is more than or equal to 30.0 percent; and the selectivity of methyl isobutyl ketone is more than or equal to 93.0 percent.