Because the high precision ultrasonic flowmeter has shortcomings of expensive, the A/D conversion circuit based on ARM for the time-difference method ultrasonic flowmeter is designed. On the basis of time-difference method ultrasonic flowmeter theory and the domestic and overseas advanced technology, a cost-effective low-power 12-bit ADC12DL040 chip and ARM processor LPC2138 are chosen for the design. According to the data acquisition system issues of the time- difference method ultrasonic flowmeter, the two-channel 12-bit low-power monolithic CMOS analog-digital conversion chip is adopted to design the nanosecond-level high-resolution data acquisition system. In this paper, the A/D chip peripheral pin circuit design is given, hardware structure diagram and software flowchart are also given. The input frequency of ultrasonic signal is 400 kHz and the sampling frequency is 8MHz.The experimental results indicated that the designed A/D conversion circuit is able to accomplish the high-speed and high-resolution ultrasonic echo signal sampling; the minimum resolution time of A/D conversion circuit is 0.305ns. The physical map of A/D conversion circuit for the time-difference method ultrasonic flowmeter is presented. The design of A/D conversion circuit for time-difference method ultrasonic flowmeter not only lower the production cost, but also lays a solid foundation for the research of low-cost and high-precision time-difference method ultrasonic flowmeter.