Abstract: In this paper,the nonlinear increasing procedure of thermoacoustical instability in Rijke tube is studied.The relations among acoustic variables are obtained by adopting the three cob ervative equations of fluid mechanics.Combining these with the nonlinear equation of thermoacoustic corelation and nonlinear reflecting condition of sound wave at the end of the tube,a method is developed to compute the increasing procedure of sound wave from little perturbation to large amplitude oscillation until the limit amplitude saturation is reached.The theory is proved by experiments.