The extra-low temperature has great influence on the mechanical properties of the material which is used widely in aerospace science and others. In our experiment of stretching the Extra-Low-Interstitial (ELI) Ti-5-Al-2.5Sn specimen, we found a new phenomenon of metal mechanics -- multi-necking. The contour of the specimen was also measured. We presented an experimental scheme of digital speckle pattern interferometry in room temperature to observe the in-plate deformation field of the whole process from the beginning of elastic deformation to the end of plastic deformation until fracture. Experimental results of the low temperature and normal temperature are much alike, expect for the difference of the multi-necking quantity. Based on the experiment results, a rough explanation was presented.