NOVELTY - Preparing small crystal grain ZSM-5/mordenite complex molecular sieve comprises (a) selecting melasilicate, silicate, and/or oxide solution of silicon as silicon source, (b) taking melasilicate and/or aluminum as aluminum source, (c) selecting ethylenediamine, ethylamine, triethylamine, n-butylamine, inorganic ammonia, tetrapropylammonium halides and/or tetrapropylammonium hydroxide as template agent M, (d) preparing solution of raw materials, (e) adjusting solution pH, (f) thermal crystallizing at 100-200 degrees C for 10-100 hours, and (g) obtaining product with grain diameter of 0.001-10 microns.    USE - Small crystal grain ZSM-5/mordenite complex molecular sieve obtained can be used in industrial production of ethylene and propylene obtained from catalytic cracking of naphtha.    ADVANTAGE - Preparation of small crystal grain ZSM-5/mordenite complex molecular sieve solves the problems of bigger grain, low catalytic activity, and fast inactivation in the reaction of ethylene and propylene obtained from catalytic cracking of naphtha.    DETAILED DESCRIPTION - A small crystal grain ZSM-5/mordenite complex molecular sieve is prepared by (a) selecting melasilicate, silicate and/or oxide sol of silicon as silicon source, (b) taking melasilicate and/or aluminum as aluminum source, (c) selecting ethylenediamine, ethylamine, triethylamine, n-butylamine, inorganic ammonia, tetrapropylammonium halides and/or tetrapropylammonium hydroxide as template agent (M), (d) preparing solution raw materials with appropriate proportion, (e) adjusting pH of solution (9-14), (f) thermal crystallizing at 100-200 degrees C for 10-100 hours, and (g) obtaining product with grain diameter of 0.001-10 microns. Mole ratio of the solution is silicon dioxide (SiO 2):(0.001-0.1)aluminum oxide (Al 2O 3):(0.01-0.3)sodium oxide (Na 2O):(0.05-0.6)M:(30-200)H 2O:(0.005 0.2)crystal seed. Molecular weight of crystal seed calculated according to silicon dioxide. Crystal seed is the amorphous object of grain containing ZSM-5/mordenite van body (80-99.5 wt.%) with SiO 2/Al 2O 3 mol ratio of 10-200 in 1-200 nm.