The invention discloses a voltage monitoring instrument of a single pile piece of a fuel cell. The voltage monitoring instrument is composed of a signal interface circuit board and a signal processingcircuit board that are in connection by direct plugging between pins and a pin base. The signal interface circuit board consists of a power conversion circuit, a signal isolation conversion circuit,an analog channel selection circuit, a decoding circuit and an impedance conversion circuit. The signal processing circuit board includes a DSP minimum system, an externally expanded RAM circuit, an externally extended AD sampling circuit, a reference voltage circuit, and a CAN communication circuit. According to the invention, with a bipolar signal isolation converter on the signal interface circuit board, one-to-one isolation conversion of the single cell voltages of the pile is carried out. The voltage monitoring instrument has characteristics of large isolation voltage, high measurement accuracy, short measurement period, strong anti-electromagnetic interference capability, wide operating temperature range, long service life, and high reliability and the like; and modularization and miniaturization of the monitoring instrument are realized.