The invention relates to a preparing method of a methanol synthesis catalyst used for a slurry bed, and belongs to the field of catalyst preparation. The method is characterized in that one or more alkalis are adopted as a precipitant and are subjected to stream combination with a copper zinc mixed solution; before stream combination, deionized water is added into a stream combining barrel, treated crystal seeds are dissolved in the water, the water is heated to 60-100 DEG C and subjected to stream combination, a prepared catalyst precursor and mother liquor are boiled, filtered and washed, conductivity of a solution at a washing end point is controlled to be not more than 10 [mu]s/cm, the catalyst precursor is prepared into slurry, the slurry is granulated and dried through spray drying, spray-dried particles are calcinated at 300-400 DEG C for 30-120 min, a proper amount of a lubricant is added into the calcinated particles, the obtained mixture is tabletted and molded with the strength of tablets is controlled, the molded catalyst tablets are smashed into particles, and an adhesive and water are added into the smashed particles and subjected to secondary molding to prepare the methanol synthesis catalyst. The catalyst is suitable for methanol preparation from synthetic gas containing CO, CO2 and H2, and is particularly suitable for a slurry bed methanol synthesis device.