A soft reduction method for a continuous casting bloom with a combination of a flat roll and a convex roll, comprising sequentially arranging a plurality of tension levelers on a continuous casting line to compression cast a casting bloom, characterized by: acquiring model data of solidification heat transfer and liquid phase cavity in continuous casting of the casting bloom according to steel grades, drawing speeds, cooling conditions, and superheat degrees for casting molding, wherein the model data include a three-dimensional temperature field profile, a two-phase region thickness, a solid-phase region thickness and a solid fraction f s along a casting direction; determining positions of rolls starting and ending reduction based on the model data, and associating the model data with each tension leveler on the continuous casting line; acquiring a volume shrinkage of the casting bloom, setting a reduction for each tension leveler roll based on the volume shrinkage, and implementing a heavy reduction operation mode on the casting bloom in a zone of the casting bloom having a solid fraction of f s = 0.9 to 1.0, wherein the corresponding tension levelers each achieve a reduction with a single-roll reduction rate of 1%-10%; implementing a soft reduction operation mode on the casting bloom in a zone of the casting bloom having a solid fraction of f s = 0.25 to 0.80, wherein the corresponding tension levelers each achieve a reduction with a single-roll reduction rate of no more than 2%; wherein the plurality of tension levelers are grouped into upstream tension levelers and downstream tension levelers, wherein the downstream tension levelers are closer to a solidification end of the casting bloom than the upstream tension levelers, wherein the downstream tension levelers are convex roll tension levelers, and the upstream tension levelers are flat roll tension levelers, and convex rolls on different tension levelers include protuberances having different lengths, and the final indentation profile generated on upper surface of the casting bloom gradually widen from bottom to top. acquiring model data of solidification heat transfer and liquid phase cavity in continuous casting of the casting bloom according to steel grades, drawing speeds, cooling conditions, and superheat degrees for casting molding, wherein the model data include a three-dimensional temperature field profile, a two-phase region thickness, a solid-phase region thickness and a solid fraction f s along a casting direction; determining positions of rolls starting and ending reduction based on the model data, and associating the model data with each tension leveler on the continuous casting line; acquiring a volume shrinkage of the casting bloom, setting a reduction for each tension leveler roll based on the volume shrinkage, and implementing a heavy reduction operation mode on the casting bloom in a zone of the casting bloom having a solid fraction of f s = 0.9 to 1.0, wherein the corresponding tension levelers each achieve a reduction with a single-roll reduction rate of 1%-10%; implementing a soft reduction operation mode on the casting bloom in a zone of the casting bloom having a solid fraction of f s = 0.25 to 0.80, wherein the corresponding tension levelers each achieve a reduction with a single-roll reduction rate of no more than 2%; wherein the plurality of tension levelers are grouped into upstream tension levelers and downstream tension levelers, wherein the downstream tension levelers are closer to a solidification end of the casting bloom than the upstream tension levelers, wherein the downstream tension levelers are convex roll tension levelers, and the upstream tension levelers are flat roll tension levelers, and convex rolls on different tension levelers include protuberances having different lengths, and the final indentation profile generated on upper surface of the casting bloom gradually widen from bottom to top.