Abstract: Traditional CAD platforms exhibit two fundamental limitations in real-time collaborative design: insufficient functionality and high synchronization latency. To overcome these bottlenecks, we present a lightweight yet highly reliable real-time collaboration plugin based on secondary development of AutoCAD. The plugin is implemented in C# and tightly coupled with the AutoCAD. NET API. An event-driven model is adopted to capture entity operations instantly. A bidirectional GUID–OBJECTID hash map combined with extended dictionaries guarantees persistent entity identification, while Protocol Buffers–based lightweight serialization and a hybrid TCP/UDP channel deliver a dual-layer framework that couples multi-modal operation awareness with incremental state synchronization. The system is hierarchically deployed in three layers—event perception, data abstraction, and transaction coordination—where asynchronous message queues and document-level transactional locks optimize concurrency. Experimental results show that, under typical Internet conditions, the plugin achieves an end-to-end synchronization latency below 30 ms and supports more than ten concurrent users editing complex drawings, significantly improving collaboration efficiency. The core module occupies only 2.1 MB. By alleviating entity-synchronization lag and data conflicts inherent in existing collaborative tools, the plugin offers a lightweight and dependable solution for real-time engineering design collaboration.