Abstract: Two groups of axioms in the natural axiom system of probability theory are set up. By the help of them the random universe will be abstracted into a causation space which is more intuitional and figurative. In the causation space, a random event is a set of cause points and the pseudo-occurence of a cause point will induce the occurence of some events. Theorem 2 proves that the set methodology in probability theory is the product of the causation space, and in this way the practice of indiscriminatingly applying the set methodology into probability theory by Kolmogorov's axiom system can be avoided.