For embedded system application status at home and abroad, this paper uses the method of compiling to design embedded Java compiler. It can directly generate the MCS-51 series microcontroller target code, does not rely on the operating system and the JVM, makes the Java language more widely applied in the low-grade embedded system, realizes the low cost industrial environment and household appliances and Internet to realize remote monitoring and control. the characteristics of Java language and embedded system are analyzed in detail, The real-time version jRate and sun of GJC compile system for reference, adopt single pass grammar translation guidance mode, the syntax analysis program unit as the core, to build an embedded Java compiler lexical analyzer, syntax/semantics analyzer and code generator, and tested the program running results and real time service quality of embedded Java compilation system.