Since it is difficult to assign weights to the importance of safety risk factor indicators in the process of safety risk assessment of informatization systems, this study proposes a safety risk assessment model based on improved D-S evidence theory and fusion weight set with a construction site as the application scenario. Firstly, the safety risk assessment process and elements of the construction site are fully studied, and a safety evaluation system for the construction site is established. Secondly, the D-S synthesis algorithm based on weight assignment and matrix analysis is used to improve the analytic hierarchy process (AHP) method, and the entropy weight method based on data is adopted to calculate the subjective and objective weights of each indicator in the index layer of the evaluation system. Thirdly, the improved D-S evidence fusion algorithm is used to synthesize multi-source evidence to obtain the indicator weights, so as to avoid the one-sidedness of a single assignment and get the optimal comprehensive weight. Finally, the comprehensive evaluation index of the construction site is calculated according to the TOPSIS evaluation algorithm. The analysis shows that the safety risk assessment model based on the improved D-S evidence theory and fusion weight set can effectively assess the safety of construction sites, reduce the uncertainty of assessment results, and improve the credibility of risk assessment results.