Perfluoroalkoxy alkane (PFA) tubes are widely used in the transportation of chemical liquids in semiconductor industry due to its good corrosion resistance and stability. In order to precisely control the dosage and time period of various liquid medicines, noncontact flow measurement of PFA tubes with small diameters is needed urgently. In this paper, ultrasonic array probe is used to excite the ultrasonic guided wave on the PFA thin tube and the flow is measured by the guided wave. Firstly, the difference of mode selection principle between ultrasonic wedge probe and array probe is compared, and the frequency selection characteristic of array probe is explained. Then, the width and thickness of array probe element are designed, and three kinds of array probe with different element spacing are prepared. The exciting and receiving characteristics of the array probe are obtained through frequency scanning. The performance of mode selection of the array probe is compared and analyzed, and the flow measurement experiment is carried out. Results show that the array probe can enhance the received signal′s magnitude and improve the measurement sensitivity. Different guided wave modes can be selected by changing the array spacing. When the L(0,5) guided wave mode is selected on the 3 mm PFA tube, the error limit of the measurement results in the flow rate range of 0~ 6. 70 m/ s is ±0. 22 m/ s.