This paper describes a new method for enhancing the fluorescence signal of a bead-based immunosensor in a digital microfluidic system by dielectrophoresis (DEP) force. A single droplet containing scattered polystyrene microbeads conjugated with fluorescent dye is first moved by electrowetting to 3-phase spiral electrodes. The fluorescent microbeads are then concentrated by DEP force at 145 VRMS over 100 kHz. It is shown that the proposed microfluidic chip can evidently enhance the fluorescent intensity even there are only very few fluorescent microbeads (10 to 100), where the droplet volume is only 6.25 nanoliter. The total operation time of the concentrating process is less than 3 minutes. The proposed signal enhancement method holds great potential for bead-based immunoassays to have better limit of detection with short analysis time and small sample volume.