The clamp-on transit-time ultrasonic flow meters are widely applied for the advantage of noninvasive measurement and convenient deployment. Its key point lies on measuring of the time difference between the upstream and downstream. While high quality of the transducer transmitting/receiving ultrasonic signals is the premise of measuring time difference accurately. This paper studies on the main factors contributing to the uncertainty of the transducer's output signal and focuses on the transducer's frequency, incident angle and the separate distance. The selection principle of transducer's frequency is presented by the analysis of the ultrasonic attention characteristics in the liquid. The incident angle range and precise installation distance of the transducers are calculated and analyzed by Snell's law. Experiments are also carried out with the 1MHz transducers clamped on the aluminium pipe of 20mm outer diameter. The results show that the accurate calculation separate distance 18.16mm is effective to guarantee the output of high quality ultrasonic wave. If the axial installation error reaches more than 3mm, the one beam of ultrasonic signal cannot be guaranteed.