Since most relative rotational speed measurement methods cannot completely meet the measurement requirements for small volume and anti-high overload in military and industrial regions, a kind of relative rotational speed measurement system based on magnetic resistance sensor was proposed. The system obtains the main frequency component in the spectrum analysis through the FFT transformation of the fixed length of time asymmetric periodic signal collected by the magnetic resistance sensor, and then gets the average rotational speed in the length of time. It can also determine the direction of relative rotation by detecting the wave-form in time domain. The system has the advantages of small size, simple structure; less constrained by the external environment, and can be widely used in various areas of the relative rotational speed measurement.