This application consists of a method for the synthesis of a type of FER/MOR composite molecular sieve. That method consisting of mixing FER seed crystals, MOR seed crystals, a silicon source, water and an acid or alkali, thus yielding a reaction mixture; by adjusting the proportions of the seed crystals added, the silicon-aluminium proportion, acidity/alkalinity and other reaction conditions, it is possible to obtain a dual phase composite molecular sieve within which the proportions of the crystal phases may be adjusted. In the synthesis process to which the method of this application relates, there is no need to add any organic template, thus reducing the cost of the reaction, in addition to reducing likely environmental pollution, thus having major potential applications.