The fluctuation of incoming and outcoming data streams of MSA(Multiplex Section Adaptation Function) in SDH is analyzed and an algorithm to generate data-monitoring signals in FIFO(First-in-First-out memory) is put forward in this paper.A MSA function ASIC design of this algorithm has been accomplished and realized by sub-micrometer process.The chip was proved to have achieved the requirements of ITU-T and have very good consequence in the simulation.