Based on the bloom continuous casting machine, the soft reduction model was established, and the change of the bloom surface and scope of reduction roll was obtained under various reduction amount and solid fraction. The results showed that: the rebound of bloom surface after reduction is about 19% of the reduction amount at certain scope of reduction amount, and the scope of reduction roll increases 16mm as the reduction amount increases 1 mm, and the rebound increases 1.7% of the reduction amount as solid fraction increases 0.2. The rebound of soft reduction should be considered when implementing soft reduction.