Objective To analyze the genetic purity of swordtail fish (Xiphophorus helleri) inbred strain for directing the breeding of swordtail as laboratory animal. Methods 29 selected single primers were used to amplify randomly genomic DNA of the inbred strain of swordtail fish. The amplified fragment bands were analyzed to calculate the similarity index and the genetic distance. Results 306 bands with the lengths between 200bp and 3kb were obtained, among them there were 29 polymorphic bands, the frequency of polymorphic bands was between 0-50.00% and the average was 9.48%. There was a high frequency of common band in different individuals. Based on RAPD patterns, the calculated similarity index within 12 samples of the inbred strain was 0.9839(0.973-0.997), the mean allelic frequency was 0.8731, and the minimum heterozygosites was 0.1269. Conclusion The inbred strain of swordtail fish has reached a high degree of genetic purity.