An unseen posterior acoustic shadow of the common bile duct is one of the signs that the common bile duct is in good condition. Unseen acoustic shadow posterior to the common bile duct is generally used in the description of biliary ultrasound findings. Acoustic shadow is an abnormal echo located after the lesion, due to the various normal and diseased tissues in the body of the acoustic energy absorption attenuation of different tissues, when the posterior wall and the posterior echo attenuation, the formation of the “acoustic shadow”, such as fibrous tissue, calcification, stones and so on, and its posterior to the formation of the “acoustic shadow”. Acoustic shadows in the posterior aspect of the common bile duct are commonly associated with stone changes in the common bile duct, which are manifested as dilatation of the common bile duct with a strong echogenic mass, accompanied by acoustic shadows in the posterior aspect. Acoustic shadowing is an important feature in the diagnosis of stones, and the absence of an acoustic shadow posterior to the common bile duct is one of the indications that the common bile duct is in good condition, and no stones or calcified foci have been found. When there is no acoustic shadow in the posterior part of the common bile duct, the condition of the common bile duct should also be judged under the guidance of a medical professional and in conjunction with other relevant examinations.