We present a new high-speed lifetime measurement scheme of analog mean-delay (AMD) method which is suitable for studying dynamical time-resolved spectroscopy and high-speed fluorescence lifetime imaging microscopy (FLIM). In our lifetime measurement method, the time-domain intensity signal of a fluorescence decay is acquired as an analog waveform. And the lifetime information is extracted from the mean temporal delay of the acquired signal. Since this method does not rely on the single-photon counting technique, the signals of multiple fluorescence photons can be acquired simultaneously. The measurement speed can be increased easily by raising the fluorescence intensity without a photon-rate limit. We have investigated various characteristics of our method in lifetime accuracy and precision as well as measurement speed. It has been found that our method can provide excellent measurement performances in various aspects. We hav demonstrated a high-speed measurement with a high photon detection rate of approximately 108 photons per second with a nearly shot noise-limited photon economy. A fluorescence lifetime of 3.2 ns was accurately determined with a standard deviation of 3% from the data acquired within 17.8 micros at a rate of 56,300 lifetime determinations per second.