In the development and application of cognitive science and ubiquitous computing, the method of building and evaluating a natural user interface has become increasingly important. In this paper, based on the definition of natural human-computer interaction, we highlight the necessity of the use of natural language between user and computers and its comprehension. Moreover, based on the semiotic triangle model of linguistics, we propose a model for natural human-computer interaction. The model can provide an explanation of how information is transmitted between the user and computers and understood. Finally, we present an approach for evaluating the user interface based on interaction naturalness, continuity, and learnability.