【Objective】 To analyze the parental origin of the extra 21^st chromosome of patients with Down's syndrome (DS) and the correlation between the partial environmental factors and DS. 【Methods】 1)The chromosomal karyotypes of patients and their parents were determined by karyotype analysis technique. 2)The parental origin of extra 21^st Chromosome in DS patients were confirmed by short tandem repeat polymerase chain reaction(STR-PCR). 3)The environment factors in parents were surveyed through the retrospective questionnaire. 【Results】 1)Among 297 DS patients, the karyotypes of trisomy, translocation and mosaic accounted for 90.2%, 5.4% and 4.4%, respectively. 2)Clinical analysis: Among 297 DS patients, there were 191 males (64.3%, 191/297) and 106 females (35.7%, 106/297), with a sex ratio of 1.8∶1 (191∶106). The patients whose age at diagnosis were within one month, within one year and within six years accounted for 23%, 63% and 92%, respectively. 3)The 21^st extra chromosome origin analysis: The genealogy of genetic information by STR linkage analysis in 22 cases displayed that the 21^st extra chromosomes of 21 cases were originated from mothers, amounting for 95% (21/22). Among them, 14 cases were caused by the first division nondisjunction of ovum in patients'mothers. Only one case was originated from the father, accounting for 5% (1/22). 4)Environmental factors investigation in parents: the age of 71 mothers who gave birth were below 35 years old, accounting for 88.8% (71/80). And only 9 mothers were equal to or over 35 years old, accounting for 11.2% (9/80) (P＞0.05). There was no statistically significant difference in smoking and drinking between the father group of DS patients and control group. However, statistical significance could be observed between the two groups in exposure to chemical materials of the parents (P＜0.05). 【Conclusions】 1)The majority of extra 21^st Chromosome of DS patients are originated from their mothers. 2)At present, age of a mother giving birth is not main factor for Down's syndrome. 3)Smoking and drinking of father, exposure to chemical materials of parents seem not to be directly related to the Down's syndrome.