The invention discloses a preparation method of molybdenum trioxide with an orthorhombic crystal form and a monoclinic crystal form. The preparation method comprises the following steps: crushing andcoarsely grinding waste silicon-molybdenum rod resistance wires into blocks, and then carrying out ball milling on the blocks to obtain nano-scale powder; calcining the ball-milled nano-scale powder in an environment without protective gas to obtain a mixture, wherein the calcining temperature is 575-650 DEG C, and the calcining time is 60-240 minutes; and soaking the calcined mixture in a mixed solution of hydrofluoric acid and nitric acid with a ratio of (5:1)-(5:3) or a mixed solution of hydrofluoric acid and hydrogen peroxide with a ratio of (5:1)-(5:3) for 60-480 minutes, carrying out washing with distilled water and absolute ethyl alcohol for multiple times, and carrying out filtering and drying to obtain molybdenum trioxide with different crystal forms. According to the preparationmethod disclosed by the invention, the waste silicon-molybdenum rod resistance wires are adopted as raw materials, the molybdenum trioxide with different crystal forms can be obtained by carrying outlow-temperature calcining and controlling the pickling time and conditions, and different catalytic effects can be generated.