The nonlinear behavior of ultrasonic absorption in glasses below 2 K is analysed by combining the two-level systems model and the infrared divergence theory. A modification of the existing tunneling two-level model of Jackie et al. is given. The Bloch equations are used as the macroscopic equations of the "magnetization" and all quantities in the equations as en-sebmble averages rather than a state expectation value. With this modification, and the defect state as a pseudo-spin in the local fluctuation field, the relation between the angle diffusion of the spin and the temperature at low temperatures is deduced. The theoretical absorption curve obtained is consistent with the experimental data. The observed T-0.7 rule of the resonance absorption under weak excitation and the proportional relation of the saturated absorption to ω2 are also explained.