Objective To investigate methods for prenatal diagnosis of spinocerebellar ataxia type 3 (SCA3).Methods Cordocentesis were performed in the pregnant SCA3 female proband of pedigree during the 20th gestational week.Polymerase chain reaction (PCR) and short tandem repeat (STR) analysis based on CEQS000 sequencer were applied to analyze the CAG repeat of SCA3 gene.Results The proband had 31/75 CAG repeat alleles of SCA3 gene; her spouse had 14/27 CAG repeat alleles; the fetuse had 14/31 CAG repeat alleles (14 repeat from the father,and 31 repeat from the mother),which is in conformity with Medelian inheritance.The fetuse inherited the normal CAG repeat allele from the mother.The above results of the fetuse were verified after its birth.Conclusion Detecting CAG repeat dynamic mutation of SCA3 gene based on umbilical cord blood and STR analysis could be a rapid and reliable method for prenatal diagnosis of SCA3.