The first spectral analysis of a spike event has been performed. It was found that the peak frequency of spectrum of the spike event on Oct. 2, 1993 drifted from higher to lower frequency for the first group of spikes, but the frequency remained almost the same for the second group. Only if the mean energy of electron beam is a monotonic or random function of time and its pitch angle is a monotonically decreasing function of mean energy,then the different observed characteristics of evolution of the peak frequency with time may be interpreted by the location of resonant circle. The spike radio emissions with one and the same peak frequency probably originated from the same region of electron cyclotron maser instability.