Background: The present study aimed to investigate the pandemic stage differences of mental health helpline help-seekers emotional responses, psychiatric symptoms, and related network structures during the COVID-19 pandemic in China. Methods: The data was collected by a large-scale psychological helpline in response to the COVID-19 pandemic in mainland China. Counselor-reported information about the help-seekers pandemic-related emotional responses and psychiatric symptoms were recorded. A total of 26,870 callers' data from February 28, 2020, to April 23, 2021, were collected in the present study. A linear probability model and network analysis were conducted to determine the differences in help-seekers mental health concerns and network structures between the pandemic (stage I, from February 28, 2020, to April 28, 2020, N = 9821) and the regular prevention and control period (stage II, from April 29, 2020, to April 23, 2021, N = 17,049). Results: Results revealed that anger, sadness, and obsession symptoms increased in stage II while symptoms of anxiety, somatization, and feelings of fear and stress were relieved. The network analysis results demonstrated both stage I and II networks centered on anxiety firmly. In stage II, the connection between anxiety and hypochondria and fear's strength centrality descended significantly. Limitations: The mental health outcomes of callers only included the counselor-reported data. Conclusions: The mental health concerns of helpline callers showed pandemic-related stage differences.