The advantages and disadvantages of nuclear instrumentat ion system of digi tal technology and analog technology are compared, and the future two development trends of nuclear instrumentation system of digital technology are proposed. One trend will be to real ize more and more functions by digital technology and become a independent system. Other trend will be to retain the basic analog circuit, and the calculation and processing function will be real ized by digital protect system. Each of these two methods has its advantages and. disadvantages, and has been adopted by the existing nuclear power plant.