With the rapid advancement of deep learning and related technologies, Autonomous Driving Systems (ADSs) have made significant progress and are gradually being widely applied in safety-critical fields. However, numerous accident reports show that ADSs still encounter challenges in complex scenarios. As a result, scenario-based testing has become essential for identifying defects and ensuring reliable performance. In particular, real-world accident reports offer valuable high-risk scenarios for more targeted ADS testing. Despite their potential, existing methods often rely on visual data, which demands large memory and manual annotation. Additionally, since existing methods do not adopt standardized scenario formats (e.g., OpenSCENARIO), the generated scenarios are often tied to specific platforms and ADS implementations, limiting their scalability and portability. To address these challenges, we propose Txt2Sce, a method for generating test scenarios in OpenSCENARIO format based on textual accident reports. Txt2Sce first uses a LLM to convert textual accident reports into corresponding OpenSCENARIO scenario files. It then generates a derivation-based scenario file tree through scenario disassembly, scenario block mutation, and scenario assembly. By utilizing the derivation relationships between nodes in the scenario tree, Txt2Sce helps developers identify the scenario conditions that trigger unexpected behaviors of ADSs. In the experiments, we employ Txt2Sce to generate 33 scenario file trees, resulting in a total of 4,373 scenario files for testing the open-source ADS, Autoware. The experimental results show that Txt2Sce successfully converts textual reports into valid OpenSCENARIO files, enhances scenario diversity through mutation, and effectively detects unexpected behaviors of Autoware in terms of safety, smartness, and smoothness.