The invention relates to a method for designing a specific instruction set processor, and the method can make the accuracy performance evaluation at the beginning of designing a chip, advance the test of a specific application, speed up the design process and reduce the re-design possibility which is because the performances of the chip can not achieve the requirements. The method establishes an LISA processor model at the early stage of the design, generates a corresponding software tool by the LISA model, builds up a virtual prototyping platform which is similar to the performances of real hardware on an ARM ESL, including a AHB bus model with precise cycle, an EMI model and an SDRAM model which are needed for designing the platform, searches out the performances under the different situations by repairing the models of the processor, obtains an optical software/hardware dividing method, directly verifies on the models and speeds up the design cycle of the processor.