Provided in the present invention are novel key intermediates of selamectin and the preparation method thereof. Also provided is a new synthesis process, using doramectin as a starting material, and obtaining selamectin via hydrogenation, oxidation, oximation and desugaring. The new process of the present invention has few steps, and is simple to operate, high in yield, low in cost and causes little pollution, and is more suitable for large scale industrial production.