Abstract: A continuous laser beam is transmitted through the burning surface of a propellant strand and converted in to voltage signal by photocell.The voltage differentiation with respect to time is proportional to the burning rate of the propellant strand.The experimental results show that the burning rate at the start of depressurization remains unchanged,and then decreased with very low frequency oscillation.During the whole depressurization process the transient burning rate is not higher than the quasi steady burning rate at the corresponding pressure and oscillates around quasi steady burning rate,except for a short time after the start of the depressurization.