Abstract: The shape of soapfilm and its characteristic and process of variation in the experiment on double funnel soapfilm are researched. An important conclusion is educed in theory and validated by experiment： the maximum distance between the two funnel is the funnel radius times 1. 325. The carelessness about this problem in some textbooks is pointed out.

Key words: experiment on soapfilm, shape, double funnel, catenary, variation calculus