Research on the construction of social group theme evolution mapping in network public opinion communication under major emergencies plays an important role in emergency response to major emergencies and maintenance of social stability. The article constructs social group theme evolution mapping based on LDA theme model, constructs social group theme heat evolution mapping based on theme timing statistics and theme frequency statistics, constructs social group theme path evolution mapping based on JS dispersion and DFS algorithms, and gives a model of the process of constructing social group theme evolution mapping in network public opinion communication under major emergencies. It is found that the model constructed in this paper can be used to analyse the characteristics of group themes, the factors affecting theme heat and the optimal communication paths of the themes, to find out the evolution of public opinion under major emergencies and to identify sensitive public opinion topics, so as to guide public opinion in a positive direction, and to provide a new reference perspective for the management of online public opinion under major emergencies.