In order to solve the problem of manually annotating large-scale corpus, this study, based on the model of Probabilistic Latent Semantic Analysis (PLSA), proposed a method of automatic emotional annotation for news comments. First of all, the "doc-topic" and "word-topic" probability matrixes were computed by PLSA model. Then, drawing upon the "word-topic" together with the ontology lexicon, the emotional categories of the topics were annotated, with the presupposition that the emotional category of words is similar to those of words within the topic which occurs most frequently. Finally, the automatic annotation was made via the "doc-topic", with the assumption that the emotional category of topics is equivalent to those of topics within the document which occurs most frequently. The experimental results showed that the accurate rate of the method proposed by this study reached about 90%.