Abstract: If the process of the welding was not controlled strictly in the P91 pressure pipebutt weld, the interlayer cracks would occur easily. The interlayer cracks were too small to be examined out if only using radiographic inspection. The reflected echo of ultrasonic tesing was too low to be examined out, thus causing misjudgement as well. Through introducing the real example of ultrasonic testing of interlayer cracks in P91 pipe butt weld field dissection, the effective method and suggestion on how to exam the P91 pressure pipe’ butt weld of interlayer cracks was proposed, that is to use the ultrasonic testing method and at the same time to use the method of radiographic inspection, and use the TOFD as well, if necessary.