NOVELTY - Preparation of 3-methoxy-4-hydroxybenzaldehyde includes mixing guaiacol and glyoxylic acid with sodium hydroxide solution to obtain guaiacol sodium solution and sodium glyoxylate; dropping sodium glyoxylate into guaiacol sodium solution, reacting, and adjusting pH; adding mesoporous copper oxide, and oxygenating; filtering, separating, recovering copper oxide, adjusting pH, and decarboxylating; and removing excessive oxidation product with saturated sodium bicarbonate solution, extracting with ethyl acetate, and rotary evaporating.    USE - Method for preparation of 3-methoxy-4-hydroxybenzaldehyde (claimed).    ADVANTAGE - The method utilizes mesoporous oxide copper having large surface area, significantly reduces amount of catalyst, does not use organic solvent such as toluene to avoid environment pollution, and it has easy separation and recycling of catalyst, and stable performance.    DETAILED DESCRIPTION - Preparation of 3-methoxy-4-hydroxybenzaldehyde comprises:    (A) mixing guaiacol and glyoxylic acid at molar ratio of 1.5-1.7 with sodium hydroxide solution to carry out conversion reaction to obtain guaiacol sodium solution and sodium glyoxylate;    (B) dropping sodium glyoxylate into guaiacol sodium solution for 2-4 hours, reacting for 4-7 hours to obtain 3-methoxy-4-hydroxyphenyl acid, adjusting pH to 1-3, and extracting unreacted guaiane with ethyl acetate;    (C) adjusting pH of 3-methoxy-4-hydroxyphenyl acid to 9-13 with sodium hydroxide, adding mesoporous copper oxide, transferring into autoclave, and purging with oxygen to carry out oxygenation reaction at 70-95 degrees C at 0.3-0.8 MPa for 3-7 hours;    (D) filtering, separating, recovering copper oxide, adjusting reaction product's pH to 1-3, and decarboxylating; and    (E) removing excessive oxidation product with saturated sodium bicarbonate solution, extracting with ethyl acetate, and rotary evaporating.    The molar ratio of 3-methoxy-4-hydroxyphenyl acid and copper oxide is 1:0.15-0.25.