Abstract: The reciprocating transmittance of sound intensity during ultrasonic testing was theoretically calculated for different frequencies and different coupling layer thickness.The influences of the thickness of coupling layer on the height of echo for different frequencies sound waves were analyzed.This result indicates that the height of echo rises when the thickness of coupling layer decreases and it will rise higher while using probes of high-frequency.The experiments of straight probe indicate that the height of defect echo rises when the pressure on probe increases and this phenomenon will be more obvious using high-frequency probes.So it is significantly important when evaluating the size of the defect to ensure that the pressure on high-frequency probes is the same as the pressure on carrying out sensitivity calibration.