The invention provides a preparation method of 6-chloro-4-hydroxypyridine-3-carbaldehyde. The final product, namely, 6-chloro-4-hydroxypyridine-3-carbaldehyde, is obtained through six-step reactions including ring closure, chlorination, etherification, reduction, oxidation and hydrolysis. The synthesis method is simple and easy to operate, a synthesis route is provided for product synthesis, theoretical and experimental bases are provided for industrial production, three wastes polluting the environment are not produced in the synthesis process, and the preparation method is very environment-friendly and suitable for large-scale industrial production. The synthesized product has high purity and yield and the production cost is low.