Different from the conventional analysis of grammatical metaphor (GM) from the perspective of syntactic variation， the paper interprets GM from the perspective of semantic variation. The semantic tension in GM is explored in relation to lexical metaphor， using onomasiological and semasiological perspectives as two complementary viewpoints on metaphorical meaning in general. In addition， the paper points out that GM is of dual restraining and emphasizing semantic features， which helps expand meaning and realize specific intention of communication. Then it is confirmed that as a metaphor grammatical metaphor helps form the category of metaphor together with lexical metaphor and is complementary with lexical metaphor.