The invention discloses a preparation method of an inoculant for a thin-wall quenched part. The preparation method comprises the following steps: 70%-80% of Si, 0.6%-1% of Sr, 0-3% of Ba, 0.9%-1.4% of Al, 0-0.7% of Ca and the balance of Fe and inevitable trace elements are mixed, melted, stirred, cooled and ground, then a product is mixed with Fe3O3 powder, and the mixture is pressed into clusters and ground. The invention further discloses an application of the inoculant for the thin-wall quenched part. With application of the inoculant for the thin-wall quenched part, the combination property of the thin-wall quenched part can be improved, and the inoculation efficiency can be improved.