By an inserted board airflow inlet distortion test, the flow will be reduced to a point with raising the inserted board. The section average total pressure and the average total pressure in the circumferential low-pressure areas are in linear relationship with the air inlet flow. Then, the circumferential distortion exponent can be estimated. The circumferential angle in the low- pressure area just is in linear relationship to the inserted board height. The section steady distortion pressure distribution can be divided into high pressure area, mixing pressure area and low pressure area, each has its own characteristic in distortion and their boundaries are obvious.