The invention relates to a decision making technique for the events, in order to solve the problem of decision-making simply and rapidly under the condition of a large amount of knowledge. The invention, for each event, can group each condition and become one group for the condition with identical prior probability and identical ' and , or ' relation to form one condition group, the corresponding relation table can be established according to the mapping relation between events, conditions and condition group, then the knowledge base for decision making is established based on the information in the relation table. A decision making application system for the events, which includes a server providing with the knowledge base and a computer connected with the server, can be established based on the knowledge base; the computer may receive one or more conditions inputted by user, the server, for every condition received, can carry out special ergodic process in the knowledge base, and look up out all condition groups with the condition, and the relative decision probability for one or more events can be figured out based on the information such as maximum probability or value of condition group, the satisfied condition number.