Abstract: For ultrasonic testing equipment, the linearity of time base directly affects the accuracy of defect location determination. Three test methods are described in the term 8. 7 and term 9. 6 of GB/T 27664. 1-2011, as well as in the term 3. 2 of GB/T 27664. 3-2012. The above-mentioned three ways were used to conduct testing and make comparison on the linearity of time base of the same digital ultrasonic instrument. The results showed that term 8. 7 is applicable to the measurement of digital ultrasonic instrument, with which the acceptance standard is within ±0. 5% of the full screen width; the term 9. 6 can be applied to the A-type pulse analogue and digital ultrasonic instrument, with which the acceptance standard is within ±1% of the full screen width; and the term 3. 2 is suitable for the ultrasonic instruments combined with detector practically used on site or in plants, with which the acceptance criteria is within ±2% of full screen width.