The invention discloses a preparation method of olanzapine crystal form I, which is as followed: take olanzapine crude purified in organic solvent to get one with HPLC more than 99.5%, and after it is resolved in dichloromethane, solid olanzapine crystal I is produced through heating enrichment or spray drying. Compared with the existing technology, the method has a substantial increase in product yield, and can effectively reduce the formation of impurity, which is a low-cost preparation method suitable for industrial production with the good purity of product.