It is known that existed causal reasoning methods will have problems to propagate the causal behaviors when there are some conflicts．To solve this problem．the authors provide a causal relation formulation．which introduces the concepts of‘time’and ‘effect’．Then a process for causal reasoning is provided：constructs the causal relation graph，then propagates the causal behaviors of the reason variables(exogenous variables)，and uses qualitative algebra to solve conflicting cases．It is an extension of 1wasaki'S causal reasoning method．