Laser fusion research has promoted the development of high power lasers. In order to evaluate the beam quality of high power Nd: glass laser system, we have measured some optical characteristics of laser beam, such as energy distribution of near-field, characteristic of far-field and divergence, wavefront distortion and coherence, spectrum of laser beam. The diagnostic instruments we used include a near-field camera for photographing the laser output energy distribution, an array camera for recording the far-field of laser beam, a radial shearing interferometer and lateral shearing interferometer with two plates for measuring the laser beam wavefront. Measurement results are obtained under the conditions of the laser output power of 1010W and l0^{11} W.