Abstract：The ultrasonic CT is used to check the post grouting at the bottom of the test piles in the being constructed Yangtze River Bridge at Nantong city，Jiangsu Province. The key of the technology is to correctly identify arrive time and to build right criterion to link sonic wave velocity and materials. There are several materials in the checked section，such as concrete，sand and grouting. The materials form several boundaries and lead to complex reflection and refraction. The signal of first arriving may be not the one along the line from ultrasonic emitter to receiver. The correct arrive times are identified by theoretic analysis and practical work. They should also be corrected to eliminate random and system errors，such as inclined drill changing distance between ultrasonic emitter and receiver. To built right criterion to link sonic wave velocity and materials，both laboratory test and in-situ practice should be combined together. The other parts of the checking process also are discussed. It is proved that post grouting at the end of pile can greatly improve the bearing capacity of pile foundation. The rigidity of mixture of cement grouting and sand varies from concrete to common sand. It depends on ratio of cement grouting and sand. If the mixture is not fully concreting at checking，the rigidity of the mixture and the bearing capacity can be further increased.
