Abstract: The semantic knowledge of natural language was analyzed and described in terms of the angle of geometric transformation on the basis of the theory of united linguistics.In fact,Translation between different natural languages is a transformation among different expressions on different languages of the same meaning.Taking the transformation of question purpose of interrogative sentence as an example,the questions of geometric transformation in united linguistics were discussed.And the interrogative sentence was classified two-dimensional again according to object and type;The transformation description language(TDL) of the six-factor form system was defined to describe its reversible transformation function and bi-directional transformation algorithm.Finally,the role of the transformation function and the application of such function in multi-language machine translation system were discussed.