Firefighters have a greater prevalence of posttraumatic stress disorder (PTSD) because of their greater risk of exposure to traumatic events. Network analysis offers novel perspectives for understanding PTSD. However, most previous network analysis studies were cross-sectional and failed to reveal the dynamics and causality of PTSD symptoms. Therefore, this study aimed to elucidate the longitudinal development of and causal relationships among PTSD symptoms in firefighters. Two survey waves were conducted among 637 Chinese male firefighters: in April 2021 (baseline) and March 2023 (follow-up). We constructed two cross-sectional networks and one longitudinal cross-lagged panel network (CLPN) of PTSD symptoms. By comparing the two cross-sectional networks, we found that the central symptoms of PTSD changed over time, with central symptoms scattered across different dimensions at baseline and focused on the avoidance dimension at follow-up. The CLPN revealed causal relationships among PTSD symptoms and showed that irritability and nightmares significantly predicted other PTSD symptoms from baseline to follow-up. In summary, our study fills a gap in the longitudinal development of and causal relationships among PTSD symptoms in firefighters and provides potential targets for subsequent prevention and intervention.