This paper gives the design process of 18 bits high-precise data acquisition system by an instance and discusses problems and practical solutions of the design. In this instance, the CPU is based on MCS-51 and the A/D chip adopts delta-sigma A/D module. It has many characteristics such as simple structure, low-cost, steady, good interface. The experiment results show that the precision of the data acquisition system reaches 18bit at 50Hz conversion ratio.