The invention relates to a method which takes kaolin as raw material and synthesizes in-situ crystal into an L zeolite molecular sieve. The method is characterized in that a part of the kaolin is calcined at high temperature to get roasting soil with high temperature, high soil is for short; the other part of the kaolin is roasted at lower temperature to get metakaolin, partial soil is for short; the two kinds of soil are mixed in a certain proportion and then is added into crystal seeds or a directing agent for the process of in-situ crystallization in an alkaline system, finally an L molecular sieve is obtained. The L molecular sieve produced by the method has the characteristic picture diffracted by X ray powder of the L molecular sieve.