The pH-sensitive organic electrochemical transistors are expected to be widely used in wearable electronic devices for in-situ physiological monitoring. However, the unclear current-voltage relationship seriously hinders it from developing in design, optimization, and application. In the present work, the current-voltage characteristic of pH-sensitive organic electrochemical transistor is investigated by combining the electrochemical equilibrium equation with the series model of differential capacitances formed at gate electrode/electrolyte and semiconductor channel/electrolyte interface. Moreover, a pH-sensitive organic electrochemical transistor is constructed by using poly (3,4-ethylenedioxythiophene)/polystyrene sulfonate as the semiconductor layer material and modifying the gate electrode with pH-sensitive polymer (poly (3,4-ethylenedioxythiophene)/bromothymol blue). The effectiveness of the theoretical model is verified by investigating the output, transfer, and pH response characteristics of the pH-sensitive organic electrochemical transistor. The experimental results show that the detection sensitivity can reach up to 0.22 mA·pH·unit^–1, and the pH response is gate-bias dependent. Then, a polynomial indicating the gate bias effect is introduced to modify the current-voltage characteristic equation. The goodness of fitting the theoretical model to the experimental results of transfer curves is found to be 0.998. The comparison between experimental and theoretical results of the gate bias corresponding to the peak transconductance and pH sensitivity responding to gate bias can also verify the effectiveness of the modified theoretical model. The results can provide theoretical support for the design and manufacture of pH-sensitive organic electrochemical transistors based flexible biosensors.