Abstract: A method is theoretically developed to determine the stability behaviour of Rijke tube with control system. The dependence of complex eigenfrequencies of acoustic modes in the tube on the amplifying coefficient and phase angle is studied. Then, the mechanism of active control of thermoacoustic instability is explained by analyzing the change of reflection of sound waves at the end of the tube. Finally,experimental investigation is made on active control. Theoretical and experimental results show that a nonlinear unstable increasing procedure originated from a linear perturbation (of acoustic quantity) can be controlled by another linear perturbation, and the system can then be stabilized. The theoretical and experimental results of this work prove the availability of actively controlling the dynamic instabilities in fluids.