When it was described in 1992, this 120 million-year-old sparrow-sized skeleton represented a new avian sharing "primitive" features with Archaeopteryx as well as showing traits of modern birds. Its basal features include, but are not limited to, a flexible manus with unguals, a footed pubis, and stomach ribs. Sinornis is known only from the type species, Sinornis santensis. The generic name comes from the Latin Sino~, 'China' and the Greek ornis, 'bird'. The specific name santensis refers to the provenance from Chaoyang county in Liaoning Province as Santa, meaning "Three Temples", is a traditional name of the county.
The holotype skeleton of Sinornis, BPV 538a-b, consists of a plate and counterplate of fine-grained freshwater lake sediment as proven by numerous fish, insect, and plant remains. The skeleton was found in the Jiufotang Formation at Liaoning, People's Republic of China. The skeleton exhibits remarkable basal features shared with Archaeopteryx, a genus of early bird that is transitional between older feathered dinosaurs and modern birds. Until the discovery of Sinornis scientists did not know much about the evolution of flight that lead to modern birds because Archaeopteryx, which lived in the Late Jurassic period around 150 million years ago, lacks many of the modern flight and perching of modern birds. Some of the primitive features found in Sinornis include moderately recurved manual unguals, as opposed to the high-recurved one in Archaeopteryx. Sinornis shares a similar pelvis with the latter genus, but its pelvic girdle has free elements unlike the fused ones found in modern birds. The iliac blades are erect and the ischium is blade-shaped rather than strap-shaped. Analogous to Archaeopteryx and older theropod dinosaurs, its pubis appears to be directed more ventrally and terminates distally in a hook-shaped "boot".
As in Archaeopteryx, the skull of Sinornis has a proportionately short, toothed snout. There are broad nasal bones that expands caudally to the external nares, with a triangular caudal margin. The dorsal and central margins of the caudal half of the maxilla run parallel while its jugal ramus does not taper caudally.
The postcranial skeleton features a separate carpus and manus in the forelimb. The manus, hand, is composed of freely articulating metacarpals, with well-formed phalanges and unguals on the first and second digits. The thorax is closed at the underside, by gastralia. The pelvis has a footed pubis.
Derived bird traits in the skeleton of Sinornis are typically flight or perching adaptations. At a time when very few intermediary forms were known, the fossil seemed to provide an early glimpse of flight evolution, showing the intermediate evolutionary step between the "primitive" wings of Archaeopteryx to specialized wings of modern birds. As in modern birds, Sinornis had a modified wrist bone, with a groove that lets the wrist bend sharply back, tightly tucking the wings during flight or rest. Sinornis was capable of flight similar to modern birds based on breastbone and shoulder structures that provided both room and support for bulky aerobic flight muscles. It also had reduced claws and small hands with a stable second finger that anchored important flight feathers. Unlike the fused finger bones of modern birds, Sinornis had separate finger bones that were well adapted for flight, while the reduction of the tail moved the centre of gravity forwards. Sinornis was also capable of perching and climbing.