As a calibration-grade instrument, the 7. 5-digit multimeter can be used for the calibration of sensor electrical parameters in the aerospace field. The basic problem of the DC amplifier directly affects the measurement accuracy of the multimeter. In this article, a low DC bias amplifier is designed, with a common-source common-base amplification circuit composed of JFET, a proportional buffer current mirror, a V-I conversion constant current source circuit as the input stage amplification circuit, and a cascaded high-gain operational amplifier to form a voltage series negative feedback structure to solve the contradiction between high input impedance and low noise amplification. The DC bias model consisted of offset voltage and temperature drift is analyzed. It is found that the offset voltage of the current mirror compensates for the offset voltage of the JFET, and the resistor repair circuit is optimized to improve the offset voltage of the JFET. Temperature drift simulation shows that the current temperature drift of the V-I conversion current source and the proportional resistance temperature drift are two main factors of amplifier temperature drift. The overall temperature coefficient of the system is improved through the selection of low-temperature drift sensitive devices, the optimization of PCB thermal layout, and the design of windproof shells. Experiments show that the absolute value of the offset voltage of this amplifier is less than 11 μV, which is better than that of the JFET precision amplifier OPA828. The amplifier temperature coefficient is -2~ -4 μV/ ℃ , which is in the same grade as the Keysight 3458A temperature coefficient without temperature calibration. The amplifier meets the design requirements of the 7. 5-digit multimeter.