Radiation field distribution is tested within a certain confined space in microwave quasi- optical focusing system of High Power Microwave(HPM) plasma experimental device. Because of the non-uniform distribution of strong electromagnetic fields in confined spaces, the traditional testing method of HPM radiation field is not applicable. Adopting non-metallic transmission device with the precise control on relative step, using the isotropic field test probe, the non-uniform distribution of the radiation field is tested. Accurate experimental data can be obtained for the study of HPM plasma with the test method.