The spatial propagation properties of output annular laser beam from an unstable resonator are studied based on Fresnel diffraction integral. According to the position of extreme value point of laser intensity on the axis, the formula used to divide the propagation space of output beam from unstable resonator into near field, middle field and far field is derived. The numerical calculation shows the variation process of output laser beam of unstable resonator from annular pattern on near field, with gradual transition of laser energy in the annular region to the beam's center, to Airy disk pattern on far field. The experimental results obtained in TEA pulsed CO2 laser with unstable resonator are in good agreement with theoretical results.