Abstract: Historic timber lounge bridges are precious architectural heritages.However,due to the combustible properties of timber,these structures remain highly vulnerable to fire hazards.To comprehensively understand the fire dynamics characteristics of historic timber lounge bridges and enhance their conservation,this study selected Yongqing Bridge—a typical cantilever-beam timber lounge bridge as the research subject.Through field investigations,the structural data and wood species of the Yongqing Bridge were obtained.Combined with experimental data from a cone calorimeter,a heat release model of timber was established.Fire dynamics simulations were conducted using the computational fluid dynamics software Fire Dynamics Simulator (FDS).The results indicate that:(1)The fire evolution of the Yongqing Bridge can be divided into six stages:ignition (0~146s),full combustion of the fire source (146~760s),flame spread (760~920s),first flashover (920s),second flashover (1 450s),and structural collapse (1 450~3 000s).The peak heat release rates during the two flashover events reached 11.6MW and 18.0MW,respectively.(2)Flame retardant treatment reduced the peak heat release rate by 16.3%,eliminated the first flashover,and delayed the second flashover by 327s.(3)After the activation of a water mist fire suppression system,the heat release rate curve showed a significant inflection point,and flames in non-source areas were extinguished within 1 800s.The synergistic application of flame-retardant treatment and water-mist systems achieves a balance between non-destructive protection and efficient fire suppression.This paper provides a reference for the research of fire dynamics characteristics and fire resistance enhancement methodologies of historic timber lounge bridges.