Abstract: Regional spillover is an important new economic phenomenon. As one of the many forms of regional spillover, regional knowledge spillover is special and invisible. The observable facts of regional interaction may become the transferring mechanism of regional knowledge spillover. To confirm this, an expanded CH model is applied in the paper to studying the relationship between regional spillover and regional interaction. The approach of anti-facts estimation is introduced to explore on the Chinese province scale data. Two kind of visible regional interaction facts, population migration of Chinese mainland inter-province and railroad trade, are employed to test the spillover mechanism between Chinese mainland provinces. The industrial familiarity and trade tightness between the provinces are also concerned to study the affection to regional knowledge spillover. The GDP and R&D expenditure are adopted to measure the regional knowledge stock, respectively. It is argued that regional knowledge spillover is not happened randomly, however it is relative to the visible facts which represent regional interactions.