This study was performed to identify changes to microbial composition after treatment with synbiotics in patients with functional constipation and to define the key microbiota in the pathogenesis of functional constipation. Fecal samples from 53 patients diagnosed with chronic functional constipation according to the Rome III criteria were analyzed using 16S rRNA sequencing. After treatment with synbiotics for 1 month, fecal samples were collected from 36 patients; after a total of 3 months, fecal samples were collected from 15 patients. The outcomes were compared with the intestinal microbiota profiles of 53 healthy community volunteers. The microbiota in the constipation group differed from that in the treatment group and healthy group. After synbiotic treatment for 1 and 3 months, the abundance of Escherichia/Shigella decreased, whereas that of Prevotella_9 and Lactococcus increased. Comparison of the microbiota among the three groups showed that Prevotella_9 was the characteristic bacteria that decreased in the constipation group and increased in the treatment group. Synbiotic treatment can improve the microbiota in patients with constipation. Identification of the key bacterial genus is important to reveal the mechanism and provide a reliable theoretical basis of synbiotic treatment. It will also promote relevant research of microbiota treatment and individualized treatments.