Abstract: The thickness of 3-D preform with closure cross-section shape braided using carbon fibers was determined synchronously using ultrasonic pulse-echo method.The transmitting principle of ultrasonic in 3-D braided preform was analyzed.The testing system and pressure mechanism consisted of computer,stepping motor,gear,leading screw and probes.It is concluded that ultrasonic attenuated seriously in 3-D braided preform because of its inner incline fiber tow structure.But with coupling water,the proper pressure value and the frequency of the probe can make the preform transmit ultrasonic wave,thus the thickness information can be obtained accordingly.The thickness value tends to be stably with the increase of pressure.However,the frequency of the probe has no notable effect on result data.