The statistical model of atmospheric turbulence was first explained briefly, the reason of image quality degradation of astronomical image through the random atmosphere was discussed. From the astronomy site-testing point of view, the descriptive parameter to the degradation of the astronomical image quality, seeing parameter r0, was discussed, the importance of seeing parameter in the site-testing was introduced, the methods used to measure this parameter, the advantage and the disadvantage of it, the history, and the actuality about development ware introduced. Then, the theory of the Differential Image Motion and the research on Portable Differential Image Motion Monitor （DIMM）was introduced in detail, the purpose, significance about research on it are defined. It not only has the advantages of DIMM, but also has so many advantages, such as, simple configuration, schlepping portability, adjusting easily, reducing Mirror Seeing effectively and so on. The total design of it and the primary parameters are introduced detailedly. At last, the errors of it ware analysed, and the method to reduce them ware introduced. It is the first time that the Portable DIMM was used to measure the seeing, and the result was analysed, and the plan of mending was putted forward.
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