The present invention relates to a preparation method for an imidazoisoindole derivative. In particular, the present invention relates to a preparation method for an imidazoisoindole derivative as shown in formula I, comprising the step of reacting a compound as shown in formula II with a compound as shown in formula III. The yield of the preparation method of the present invention is greatly improved compared with the prior art, and the purity of the product is also higher such that the subsequent reaction is easier and the post-treatment operations are reduced. The preparation method is more suitable for industrial scale-up productions.