TTCN-3 (Testing and Test Control Notation Version 3) is a formalized testing description language, which is widely used in protocol testing and other testing fields. As TTCN-3 test suites are more and more complex, it becomes difficult for testers to locate the place which causes testing failures without debugger's help. This paper presents a non-intrusive TTCN-3 debugging method which uses third-party debugger to debug TTCN-3 complier generated target code. In addition, the method integrates statement mapping and symbol analysis algorithms to implement basic debugging functionality. Experiments show that this method imposes little influence on testing system and has a better performance while debugging large-scale test suite.