The invention discloses a wellhead reset device and reset method suitable for a bottom flange. A wellhead reset device comprises a hoop, liquid cylinders, a double flange short piece, and a blowout preventer, the hoop is composed of two halves of the hoop, the two liquid cylinders used for remotely controlling the expanding or retracting of the hoop are symmetrically arranged at the two ends of the hoop, liquid cylinder rods at both ends of the liquid cylinder are connected with the two halves of the hoop correspondingly, the two halves of the hoop are coupled to form the hoop through the twoliquid cylinders, the upper end of the double flange short piece is fixedly connected with the blowout preventer in a sealing mode, the lower end is used for connecting with the wellhead, and the hoopis fastened on the double flange short piece. According to the wellhead reset device and reset method suitable for the bottom flange, scrambling mounting of the wellhead can be operated remotely in the whole process, the working efficiency of scrambling mounting of the wellhead is improved, and the risk of a rescue person approaching the wellhead is lowered.