In commissioning the HIRFL-CSR(Cooler Storage Ring of Heavy Ion Research Facility in Lanzhou)

with the spectrum analyzer monitoring the beams on-line

we measured many parameters of the beam status for improving quality of the heavy ion beams.In this paper

the sudden change of the beam center frequency is analyzed.A theoretical analysis is done mainly on instabilities of the electron voltage and the neutralization factor.Simulations were made for the problem.According to the simulation result

we can conclude that it is the instability of neutralization factor that caused the problem.