An XML (extensible markup language) parser is the fundamental software for analyzing and processing XML documents. The implementation of a high performance full-validating XML parser is studied in this paper. This research develops a OnceXMLParser which supports three kinds of parsing models. It passes the rigorous XML conformance testing and API (application programming interface) conformance testing. OnceXMLParser adopts light-weighted architecture and is optimized on many aspects including efficient lexical analysis, statistical automaton implementation, reasonable resource allocation strategies and some fine tunings on language level. Performance testing results show that OnceXMLParser has outstanding parsing efficiency.