Provided is an online cooperative control method for soft reduction and heavy reduction in bloom continuous casting. A quantitative relationship of a theoretical ultimate reduction amount to a reduction position is accurately determined according to an offline three-dimensional solidification and heat transfer model for a continuous casting bloom and a three-dimensional reduction model for a continuous casting bloom under thermo-mechanical coupling; a thermal state of a continuous casting bloom, soft reduction and heavy reduction zones, and a total reduction amount are calculated in real time with an online three-dimensional solidification and heat transfer calculation model for a continuous casting bloom; actual reduction amounts of the rollers for soft reduction and heavy reduction are calculated and distributed online based on a real-time equivalent central solid fraction and a theoretical ultimate reduction amount of each reduction roller. Online cooperative control of soft reduction and heavy reduction in bloom continuous casting is realized.