The opal mutation (UGA) at nucleotide 2670-2672 in the replicase gene of the attenuated tomato mosaic virus K (ToMV-K) mainly contributes to the virus attenuation based on a series of studies on the viral attenuation mechanism. From analysis and comparison between the replicase gene mutation point of ToMV-K and the related regions of all plant viruses containing the leaky UGA, we have found that some characters, including the CGG motif, alpha-helix structure and some specific amino acids, are, presumably, able to helpfully confer the readthrough mechanism. Finally, some other ssRNA plant viruses like PVX, PVY, CMV have been analyzed. We found that their genomic modifications and viral attenuations could be explored according to the mutation mode of the ToMV-K replicase.