In this paper, a portable potentiostat with a standalone signal generator is proposed for the signal processing of vanillylmandelic acid (VMA) biosensors. The proposed potentiostat primarily consists of two microprocessors: one is used to design the programmable waveform generator, and the other is used to measure the current of the biosensors. It can perform general electrochemical analysis functions, such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), amperometry (IT), and potentiometry (E). In the experiment, we adopt a VMA biosensor to verify the performance of the proposed potentiostat. The experimental results show that the proposed potentiostat has the merits of good accuracy, larger range of detected sensor current, low cost, and high portability.