Causal qualitative reasoning is aimed to explore the causalities betweencomDonents in a system by analyzing the descriptions for physical behaviors and relation-shiDs of the system．This paper presents a model and algorithms of causal qualitative reasoning based 0n constraint and variable analysis．The result can be widely applied to many aspects of product design systems，such as parametric design by geometric reason-ing，decomposition of complex design tasks and dependency analysis of design parameters.The algorithms have advantages in applicability，efficiency，robustness and ability to deal with 11nder-constrained conditions in a constraint system．