Abstract: Fluctuation of the producing fluid level is strong for the influence of foam segment in well.So that admission information can not express the liqiud level of pumping well correctly.The flow State of liquid in the tubing-casing annular space was analyzed.The article proposed the theoretical calculation formula of the foam surface.In another words,the pseudo foam segment is taken off,so we can get the correct and reliable liquid level data,which provides dependable information for the measures of pumping well.