Abstract:Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important pathogen that severely impacts swine industry worldwide， which causes reproductive failure in sows and respiratory disease in growing pigs. PRRSV is divided into genotype 1 and genotype 2， with characteristics of easy variation and evolution as well as diversified strains and pathogenicity differences. The genetic variation of PRRSV genome is mainly distributed in its nonstructural protein 2-coding region and structural protein genes ORF5 and ORF3. The recombination among different strains is one of the major mechanisms contributing to the variation and evolution and emergence of novel strain of PRRSV， and meanwhile innate and adaptive immunity of pigs play important roles in accelerating its variation and evolution. A better understanding of the nature and mechanism associated with the variation and evolution of PRRSV will be helpful for prevention and control of its transmission and prevalence.