Reflective middleware is the major approach to improving the adaptability of middleware and its applications. Current researches and practices pay little attention on the usability of reflective middleware. There is also lacking a systematic way to adapt a runtime system via reflective middleware. This paper presents the design and implementation of PKUAS (Peking University Application Server), an architecture-based reflective component operating platform compliant with Java 2 Platform Enterprise Edition. PKUAS constructs and represents its platform and applications from the perspective of software architecture so as to provide an understandable, user-friendly and systematic way to use reflective middleware.