Abstract: The weak magnetic field measurement is one of the important means to study the material properties and to explore the unknown world. For the magnetometer with analog signal output, an A/D converter is needed to support a high-precision signal acquisition system for the high-precision measurement of weak magnetic field signals. Therefore, a signal acquisition system for high precision weak magnetic field is developed based on the Sigma-Delta A/D converter. The system hardware circuit design and the functional design of the FPGA as the system core controller ensure the Read/Write control of the A/D converter by the FPGA, as well as the receiving and the transmission of the ADC output data. Results of the performance test show that the effective resolution of the system can approach the theoretical value of 21bits, with an output RMS noise as low as 1.9 μV and a measurement precision of 0.1 nT, better than the required value.