Abstract: A fast adjustable dumbbell is provided, which includes a handle provided with a first accommodation cavity; a plurality of dumbbell pieces, two groups of dumbbell pieces are respectively provided at outer sides of two ends of the handle, and the dumbbell pieces are provided with first through holes; a rotatable shaft including two independent half shafts, which are provided in the first accommodation cavity, the two half shafts can move relative to the handle, and outer surfaces of the two half shafts are provided with symmetrical first threads; two transmission components respectively fixed at the two ends of the handle, and the transmission components are provided with second through holes corresponding to the first accommodation cavity. Hole walls of the second through holes are provided with second threads that are engaged with the half shafts; when the handle is rotated, the two transmission components are driven to rotate.