The invention provides a method and a device for extracting scale-invariant features of images in video streams and a method and a device for recognizing objects on the basis of the method for extracting the features. The images comprise gray images and corresponding parallax images, correspond to moments t1 in the aspect of time, and include precedent images and posterior images in the video streams in the aspect of time. The method for extracting the features can include positioning critical points in the images; generating description regions around the critical points, and describing each description region around the corresponding critical point in four dimensions x, y, z and t; generating descriptors for each critical point on the basis of the description region of the critical point. Values of each description region in the range of each of the corresponding dimensions x, y, z and t are not equal to zero. The descriptors are used as the scale-invariant features of the critical points. The methods and the devices have the advantages that information of time domains, information of depth domains and information of image planes are closely combined with one another, so that the four-dimensional scale-invariant features can be extracted, and the methods and the devices are suitable to be applied to machine learning.