As a linguistic theory, systemic functional linguistics has a great influence on many aspects. The nature of language and the related theories of systemic linguistics can be used to analyze the problem of language selection in the conclusion part of the thesis. The three major theoretical functions of language: conceptual function, interpersonal function and discourse function are used to analyze the differences and similarities between the conclusions of the three subjects. Taking the legal, physical and mechanical engineering language database as an example, the application of the sentence pattern of "noun +that" in the conclusion part is analyzed. The results show that social science tends to construct knowledge in light of different opinions and viewpoints in this field, while natural science and engineering disciplines usually accumulate knowledge based on experimental observation and evidence.