The invention discloses a primer and a kit for detecting low-frequency mutation of a target gene. The invention provides a set of primers for detecting a mutation condition of a region to be detectedof the target gene of a sample to be detected and the primers comprise a Barcode primer F1, an upstream primer F2, a downstream outer primer R1 and a downstream inner primer R2. The set of primers, designed by the invention, are used for constructing a library; the primers can be used for detecting a tissue sample and also can be used for rapidly, conveniently, sensitively and specifically carrying out target amplification on different regions of free DNAs in samples including blood, urine, cerebrospinal fluid and the like, and mutation which is as low as a 0.1 percent level is efficiently detected; the experiment operation is greatly simplified, the library loss and pollution are effectively avoided, the cost is remarkably reduced and the efficiency is improved.