Xu Xing described Sinornithosaurus and performed a phylogenetic analysis which demonstrated that it is basal, or primitive, among the dromaeosaurs. He has also demonstrated that features of the skull and shoulder are very similar to Archaeopteryx and other Avialae. Together these two facts demonstrate that the earliest dromaeosaurs were more like birds than the later dromaeosaurs were.
Sinornithosaurus was among the smallest dromaeosaurids, with a length of about 90 centimetres (3.0 ft). In 2010, Gregory S. Paul gave higher estimations of 1.2 metres and three kilogrammes.
In the mediaSinornithosaurus makes an appearance in the episode Feathered Dragons in Planet Dinosaur, where it is first shown to prey on a smaller relative, Microraptor, but fails. Sinornithosaurus is later shown to have a poisonous bite by comparing its teeth to the Gila Monster lizard. A pack of Sinornithosaurus have taken down a Jeholosaurus with the help of their poison.
Lately, the theory of Sinornithosaurus being venemous has been doubted, or even disapproved by the paleontologists.