Objective: To analyze the mutation situation and characteristics of 21 STR loci in Wenzhou population. Methods: A total of 21 200 samples from 9 418 paternity testing cases supporting parental relationship were collected and amplified with AGCU EX22 human fluorescent labeled STR multiplex amplification kit. The mutation rate of each locus and the source, fragment size, mutation steps and the increase or decrease of repeat units of mutant alleles were counted, and the correlation between mutation factors was analyzed. Results: In 9 418 paternity tests, 314 mutations cases and 319 mutation events were observed. Mutation was observed in 20 loci out of 21, with the mutation rate ranging between 0.08‰ and 3.48‰, and there was no significant difference in the mutation rate among each locus (χ2=29.77, P>0.05). The ratio of paternal mutation to maternal mutation was 4.78:1, and the mutation probability of paternal allele was generally greater than that of maternal allele, showing statistical difference (t=4.55, P<0.05). There was no significant difference in the increase,decrease and uncertainty of repeat units between paternal mutation and maternal mutation (χ2=0.28, P>0.05). There was significant difference in the increase, decrease and uncertainty of repeat units between one-step mutation and two-step or more mutations (χ2=7.22, P<0.05). There was statistical difference in the increase, decrease and uncertainty of repeat units in short, medium and long segment allele mutations (χ2=26.81, P<0.05). Conclusion:The allele mutation of STR locus is common, which should be paid attention to in the paternity test and the construction of DNA family search database.