The purpose of this study was to suggest the measures and methods for securing the stability of temporary bridges by analyzing the cause for the collapse of the temporary bridge built for the construction of the GunNam flood control reservoir located at the main channel of the Im-Jin River. Numerical simulations (one-, two-, and three-dimensional) were performed by collecting field data, and the results showed that the collapse occurred because the height of the temporary bridge was lower than the water level at the time of the collapse. Also, the drag force calculation showed that when the guardrail installed on the upper deck structure was not considered, there was no problem as the calculated values were lower than the design load, whereas when the guardrail was considered, the stability was not secured as the calculated values were higher than the design load, 37.73 kN/m. It is thought that the actual force of the water flow applied on the bridge increased due to the accumulation of debris on the guardrail as well as the upper deck.