By combining a digital counting processing method and the computer sampling technique, a real-time signal processing system is realized to construct a laser sweeping technique (LST) with real-time ability to measure the instantaneous closing velocity of mechanical heart valve prosthesis. After a brief review of LST, the effect of the positions of the collimating lens and the moving grid on the measurement results is analyzed. A practical grid construction is presented, and the principle and the construction of the signal processor are discussed in detail. The statistical behavior of MHV closure can be drawn out from real-time realization of instantaneous closing velocity obtained by using the new laser diagnostic system. The system has been used for the diagnostic of leaflet closing velocity of a monoleaflet type MHV, and the experimental results are given.