Whether the negation is ambiguous or univocal has not yet been determinated. Some studies of monoguist approach argue that the ambiguity of negation cannot be justified unless there are marks of metalinguistic negation. In order to prove the invalidity of this argument and to support the pragmatic ambiguity of negation, taking shi in Chinese as an example, this paper proves in the framework of metarepresentation that some linguistic forms often used in metalinguistic negations are not particular marks of metalinguistic negation. With the theory of three domains, this paper proves that the pragmatic ambiguity of negation does not exist between metalinguistic negation and descriptive negation, but in the three cognitive domains. In order to identify the cognitive domain in which the negation is located, some metalinguistic negation requires a special linguistic form, whose existence should not overthrow the ambiguity of negation but prove the pragmatic ambiguity of negation.