Abstract: The Wangzhuang oilfield is a rare heavy oil reservoir of strong water sensitivity at home and abroad. Expansion of clay mineral seriously harms the exploitation. In order to prevent the problem,three expansion-preventatives of KOH,NH₂CONH₂ and FGW-2 have been chosen in the indoor experiment which includes static compatibility and core mobility experiments. These experiments have achieved excellent effects. But NH₂CONH₂ (urine) is the best in the three preventatives because it can increase permeability of water phase by 144%.The basic mechanism for that is that NH₂CONH₂ can transform expansion clay minerals into other mineral of non-expansion. The effect can be comparable with the U. S patent 4,227, 575 report. This study is to use the experiment data and to explore expansion-prevention effect and mechanism.