In order to solve the problem of lack of effective failure mode and defect propagation analysis method for intelligent software, this paper first proposed a novel failure mode and propagation analysis method for intelligent software based on causal relationship, and then presented the defect propagation mechanism analysis method for intelligent algorithm, including multi-source perception of defect data, defect propagation path identification based on dynamic causality diagram, defect root cause mining. Finally, case study was carried out based on a target-detection intelligent software with YOLOv5 algorithm. The effectiveness and feasibility of the proposed methods were verified.