This letter investigates electrophoretic molecular communication (EMC) operating in circular duct channels. EMC utilizes the time-varying electrophoretic force that can controllably induce the movement of charged particles to enhance communication performance. In circular duct channels, where the memory component is high, intersymbol interference (ISI) must be considered. Thus, this paper presents a method to design an electric field under the framework of the calculus of variations, simultaneously reducing the ISI and strengthening the information signal reception. The numerical results show that the proposed electric field can significantly reduce the bit error rate compared to the constant field benchmark.