Abstract: A significant safety incident occurred during the installation of a two-sided supported regular quadrangular pyramid-shaped grid truss in a project， resulting in multiple fatalities and injuries. To investigate the cause of the grid truss fall and reduce the recurrence of similar long-span spatial steel structure accidents， the methodology of “hypothesized causes-theoretical analysis-model verification” was adopted. Firstly， through on-site investigations and in conjunction with project design documents， construction sequence， and structural analysis， the following three hypotheses regarding the cause of the grid truss collapse were proposed： 1） during the high-altitude piece-by-piece assembly process， the self-weight and construction load of the cantilevered extension section of the grid structure were imposed on the completed edge strip， causing the completed edge strip to be overloaded； 2） the upper chord of the grid truss was not connected to the steel column， and both the top and bottom of the steel column were hinged， invalidating the portal frame system and rendering the overall structure unstable； 3） the effective connection between the end joints of the upper chord was not established， and the vertical load was transmitted through the smaller vertical web rod， deviating from the construction drawing of the grid truss. Secondly， to validate the above hypotheses， a finite element model for the construction simulation of the grids truss was established using the spatial steel structure analysis software 3D3S. The boundary conditions， construction steps， and actual circumstances of the model were identical. The analysis of the vertical deformation and axial force variations of the support column during the construction process of the grid truss was carried out. The results indicated that the construction scheme altered the load transfer path of the grid truss， and the axial force on the support column during the construction process far exceeded its stable bearing capacity， causing the column to buckle under compression and lose its supporting capacity， which resulted in the overall collapse of the grid truss. Finally， based on the tragic lessons learned from the accident and considering the complexity of forming spatial structures， the necessity of conducting construction simulation analysis was emphasized. It was pointed out that such analysis， which constitutes a virtual construction process， can replicate the sequential installation of components， the evolution of support constraints， and the order of load application. This process not only helps identify defects and risks in the construction scheme but also reveals deviations in the internal forces and deformations of components， which are caused by the construction sequence and differ from those in a monolithically formed structure. These insights allow structural engineers to collaborate with construction designers， compare results， and determine whether the deviations are acceptable， thereby ensuring the safety and rationality of the construction scheme.