Abstract: This paper presents a design of softdog based on FPGA and describes hardware and software design.PC applies the CP2103 to transfers data via the USB to the FPGA device without modifying the application.The SPARTAN XC2S200 FPGA is the core of the softdog.It completes acquiring, processing and sending data.Taking the AES as Encryption algorithm, the design can be working at 255.875MHz using 1520 slices to encrypt data.This method is suitable for the application of the low-cost encryption chip that the data throughput is not critical.