Abstract: The failure cause of a reducing unit pipe was analyzed by optical microscopy and scan electronic microscopy. It is found that the failure mode of the pipe is early fatigue fracture. Before solidification, the carbon cloth bended at the edge of the adapter sleeve, resulting in delamination of carbon cloth and fracture of part carbon fiber. As a result, the properties of this area declined significantly after solidification, resulting in fatigue facture under the effect of fatigue load.