Taking the explosion of a shipyard as an example, the investigative techniques had been comprehensively utilized (such as on-site investigation, interview, numerical simulation, theoretical calculation, trace analysis, etc.) to conduct in-depth research on the development of the accident, consequences and mechanism of the explosion. An on-site survey of the involved hull revealed fresh welding slag remaining on the starboard main deck in the area of the No.7 empty cabin utility hole. In addition, the empty compartment No.7 on the starboard side was identified as the explosion's origin based on the ignition source traces, the extent of the hull damage, and the cracking direction. The root of the explosion accident was restored according to results from a comprehensive on-site investigation. The paint and thinner would volatilize and produce massive organic combustibles during painting operations, forming explosive mixtures when mixed with air and accumulating in the empty compartment's limited space. The explosive gas mixture contacted spattered weld slag at the utility hole, causing an explosion in the empty compartment No.7 on the starboard side, which triggered explosions in the remaining compartments. As a result of the breeding-development-dissipation of the shock wave, the explosion first intensified and then gradually reduced damage to the surrounding of the NO.7 empty cabin as the center. The simulation with the CFD analysis software FLUENT revealed that the explosion mixture diffused 10% outward through the utility hole in 12 h. According to the diffusivity analysis results, the gas mixture's volumetric concentration was calculated to be 7.3%, which is sufficient for a combustion explosion. The equivalent amount of TNT for the explosion was 188 kg, which was inverted to 203 kg depending on the extent of damage to the buildings at the accident site after the explosion. It is in good agreement with the TNT equivalent of the explosion obtained from calculations based on the physical parameters of the gas mixture, which proves the practicability of the calculation.