The invention relates to the field of spinning, and discloses a preparation method of multi-irregular composite nylon 6 fiber. The method comprises preparation of fully drawn yarn (FDY) and preparation of polyester pre-oriented yarn (POY). The preparation of the FDY comprises the following steps: (1) mixing; (2) spinning; (3) drawing. The preparation of the POY comprises the following steps: (1) mixing; (2) spinning; (3) guiding yarn. The multi-irregular composite nylon 6 fiber is prepared by merging the prepared FDY and the prepared POY on a guide roller No. 3, then compounding the mixed fiber on a subpackage type interlacer, and finally winding by using a winding device. By reforming and optimizing the existing production line for the nylon 6 fiber, the comprehensive compound modification such as multiple components, multiple functions, various colors and multiple irregular cross sections for the nylon fiber can be realized while not influencing the spinnability of fiber production, the types and styles of fiber and fabric can be greatly enriched, the application space and field of the fiber are expanded, and the application field of the product is improved. The preparation method is high in efficiency, low in cost and good in stability.