Objectives We aimed to analyse weight change and risk factors associated with excessive weight gain in preschool children during the COVID-19-related lockdown in western China. Methods A retrospective observational study of eight randomly selected kindergartens was conducted. Data was collected via online electronic questionnaires during the lockdown. Results During the COVID-19 lockdown period, the incidence of overweight and obesity among preschool children involved in the study was 7.6 and 14.7 %, respectively. In addition, the incidence of obesity among children aged 3–4 years was 26.3 %, which was higher than that of other age groups. Children with excessive weight gain (weight gain ≥1.0 kg) spent more time on TV and video viewing during lockdown than children with normal gain. Among children with excessive weight gain, weight before lockdown and fathers’ BMI were higher than those of children with normal weight gain. Heavier weight before lockdown (OR 1.044, p<0.05), higher father’s BMI (OR 1.022, p<0.05), fresh fruit consumption during lockdown (frequently, ≥5 times/week) (OR 5.946, p<0.05), and long touch-screen device time during lockdown (OR 1.259, p<0.05) were found to be risk factors significantly associated with excessive weight gain. Living space (80–100 m 2 , OR=0.499, p<0.05; 100–150 m 2 , OR=0.467, 95 % CI 0.26, 0.83) and good mental behavior during the lockdown (OR=0.056, p<0.05) were found to be significant protective factors against excessive weight gain. Conclusions There was a clear trend in excessive weight gain among preschool children during the COVID-19 lockdown, especially those who had already had heavier weight before the pandemic. The obesity-promoting environment resulting from the COVID-19 pandemic may lead to a further serious exacerbation of the childhood obesity pandemic. Therefore, child health care providers must pay close attention to this and implement effective preventive measures in a timely manner.