In this paper, we design and implement a low-cost and standalone potentiostat for the signal processing of electrochemical biosensors, and further exert the vanillylmandelic acid biosensing to verify its performance. In the experiment, we adopt a personal digital assistant (PDA), rather than using a personal computer (PC), to set the experimental parameters to the potentiostat through Bluetooth wireless communication. The biosensor’s measurement is controlled by a mixed-signal microprocessor, and the measured data are stored in a memory card. The power supply of the standalone potentiostat is provided by a USB charger. When the experiment is finished, the data stored in memory card can be retrieved and analyzed by a computer. Our experimental results show that the proposed potentiostat has the merits of good accuracy, low cost, low power consumption, and high portability.