The invention discloses a bubble generator for flexibly regulating the bubble size, and relates to the field of bubble generators. The bubble generator comprises a box, a bubble generation tube, a sleeve joint, a gas-phase input joint and a liquid-phase input joint. The box is of a rectangular body with an inner cavity. The bubble generation tube is an elongated metal hard tube. The first end of the sleeve joint is fixedly mounted on the upper end surface of the box, an inner cavity of the sleeve joint is communicated with the inner cavity of the box, and the second end of the sleeve joint isfixedly connected with the first end of the bubble generation tube. The gas-phase input joint and the liquid-phase input joint are hermetically and fixedly connected with the lower end surface of thebox, and an inner cavity of the liquid-phase input joint is communicated with the inner cavity of the box. The bubble generator has the advantages that the bubble size can be flexibly regulated without change of main current flow, bubbles are evenly generated, and flexible combination according to needs is achieved.