How to migrate binary code between different ISA efficiently is a difficult problem in binary translation. This paper analyzes this problem at the aspect of register mapping and an innovative register mapping method combining segment mapping and function cutting out of special-purpose register has been presented in this paper. An implementation of the method has been done on trend, which is a dynamic binary translation system translating and executing binary code of PowerPC on Alpha. The testing results of NPB-serial and SPEC2000 show that the method can simplify instruction translation, decrease code expanding and improve the execution efficiency of binary code obtained after binary translation evidently.