Trisomy X (47, XXX) is a sex chromosome aneuploidy condition in which females have an extra X chromosome, compared to the 46, XX karyotype in typical females. There is considerable variation in the phenotype, with some individuals very mildly affected and others with more significant physical and psychological features. However, the trisomy X in this case, without any of these phenotype, is rarely reported. Here, we report a case found during DNA sample collection in a study of genetic polymorphism analysis of loci in Chinese ethnic group, of a female with neither laboratory or clinical signs of Triple X syndrome. She was born at her mother's 60years old and her father's 62years old. Advanced maternal age was found acting as a significant risk factor of Triplo-X. Moreover, her child are also born without manifestations of 47, XXX syndrome. Pedigree study demonstrated the normal karyotype of the children. A diagnosis of 47XXX was made on the basis of a chromosomal study. Therefore, laboratory investigations (including PCR amplification, more than two kinds of X-STR genotyping, G-banding karyotyping analysis and Pedigree study) are applied to rule out the possibility of Mosaicism (45, X0/47, XXX) and ascertain her 47XXX karyotype without mosaic. The objective of this study was to report a case of trisomy X, diagnostic investigation and management of the case, and to analysis the genetically possible reasons behind the case. To our knowledge, this case is a rare one, found in DNA sample collection for the estimation of gene frequency in the process of genetic polymorphism study, of non-mosaic 47, XXX without signs of physical syndrome and born healthy children. In this study, it revealed that the proportion of trisomy X would be more than official statistics and risk of systemic disabilities is lower than estimated. Moreover, we found out that sample mixture and mosaicism act as the interference factors in forensic test. Therefore, we draw the conclusion that attentions and certain improved methods should be applied to the diagnosis of non-mosaic triple X, which is of great significance in decreasing the interruptions in the whole process of forensic and paternity identification.