The invention discloses a preparation method for selectively synthesizing a pure-phase MCM-48 (Mobil Composition of Matter No.48) molecular sieve by using a sol-gel method. The preparation method comprises the following steps: I, dissolving a template agent into deionized water, after the template agent is completely dissolved, adding ethylene glycol, and stirring for 10 minutes so as to obtain amilky white mixed solution A; II, putting absolute ethyl alcohol and ammonium hydroxide into the mixed solution A at a stirring state, and stirring for 5 minutes so as to obtain a mixed solution B; III, putting tetraethoxysilane into the mixed solution B at the stirring state, and continuously stirring for 24 hours so as to obtain a milky white solution C; IV, performing suction filtration and washing on the solution C so as to obtain a mixed solution D; drying for 12 hours at 60 DEG C so as to obtain a white solid; V, grinding the white solid, sub-packaging in a plurality of crucibles, and calcining the crucibles in a muffle furnace, thereby obtaining the pure-phase MCM-48 molecular sieve. By adopting the preparation method, not only are preparation procedures simplified, but also the molecular sieve preparation cost is reduced, and meanwhile microscopic structure of the molecular sieve is regulated and controlled.