As a single high-speed camera can not meet the requirements of high-speed target measurement for large fields, high frame rates, high resolutions and large volume data processing，the high speed target measuring technology completed by external field stitching was researched. On the basis of analysis and comparison of external field stitching methods, a ultimate scheme of external field stitching with external fields from four cameras was proposed. In the scheme, two cameras were mounted at one side along the horizontal axis of a 'T' type tracking frame and the others were at the other side. A prototype of external field stitching measuring system made of four lenses with 10°field-angle was designed,and its actual stitching field of view is 40 ° (horizontal) × 10 ° (vertical).When the parallax is 360 mm, the complete image can be obtained for the target over 247 m away from the cameras without blind spots.