Integrating the nature of science (NOS) into textbooks is one of the challenges in science curricula. This case study assessed the issue based on a sample of high school biology textbooks in Mainland China. We quantitatively compared changes in the extent, quality, and distribution of NOS representations between 2004 and 2019 editions. Three main findings emerged: (i) NOS representations have improved in the 2019 editions compared to the 2004 editions, with a 42% increase in frequency and a score growth rate of 70%. (ii) There was an imbalanced distribution of NOS aspects, focusing on 'empirical' and 'tentative' aspects and omitting 'theory-laden', 'myth of 'the scientific method', 'scientific theories', 'scientific laws', and 'social and cultural embeddedness in science aspects. All NOS aspects were heavily weighted towards the genetics theme. (iii) NOS in the 2019 editions is still addressed as it was in the 2004 editions, with unsolved aspects omitted yet again or addressed inadequately. These results constitute direct evidence for future textbook revisions.