Paul Sereno et al. (2001) considered a similar prehistoric bird species from the same formation, Cathayornis, to be a junior synonym of Sinornis. They interpreted the anatomies of the two as very similar and sharing key autapomorphies of the pygostyle.
However, also in 2001, Zhou and Hou continued to distinguish Cathayornis from Sinornis by the former's larger size, a shorter, straighter, finger number I, with a slightly longer claw (ungual), the absence of an atitrochanter, and other features. A paper describing a second species of Cathayornis in 2008 by Li et al. also considered the genera to be distinct.
The first thorough review of Sinornis and Cathayornis was published by Jingmai O'Connor and Gareth Dyke in 2010. O'Connor and Dyke concluded that despite the 2001 paper by Sereno and colleagues, the two birds were not synonyms and in fact differ in several clear ways, including different proportions in the wing claws and digits, differences in the pelvis, and size of the pygostyle.