「流動性」是指交易媒介轉換成其他物品的速度，或方便的程度。較之實物，金屬鑄幣(含金銀銅等)具有更高的流動性。故物物交換模式之困難，乃因其流動性太低。經濟體中貨幣數量是流動性的基礎。但流動性過剩造成通貨膨脹，過少則造成通貨緊縮。因之貨幣數量的劇烈變動(反映在緊縮與膨脹上)，意味著將對經濟造成流動性的衝擊。\r 對於中古六朝歷史，全漢昇最先提出「自然經濟」之說，其後學者補充指出魏晉南北朝是「貨幣大混亂」時期，勞榦則進一步言，中古貨幣的問題在於幣制之混亂，不能為市場交易提供「精確的標尺」。本文則進一步從「流動性」概念出發，提出「兩個悖論」之說，認為中古經濟受到因通貨供給不足，以及多次發行虛價錢幣、頻繁改制、錢幣剪鑿和優質銅錢遭到窖藏這些因素的影響，面臨到通貨的膨脹與緊縮同時存在、彼此交織的「雙重的流動性困境」；這種困境表現出「長期通貨緊縮格局下更迭發生膨脹性擾動」的特徵。這一個現象不僅有文獻上的證據，也受到眾多考古材料的支持。三國兩晉南北朝雙重的流動性困境源頭主要在於各朝政府不當的貨幣政策。此一困境成為決定中古社會經濟結構的重要力量之一。 “Liquidity” refers to the speed and convenience in which a asset can be exchanged to obtain other goods. Compared with other objects, the metal-cast (gold, silver, copper etc.) coins have the best liquidity. Difficulties in bartering are a result of poor liquidity. The quantity of money is the foundation of the liquidity in any economic system. “Liquidity surplus” results in the inflation and “liquidity shortage” in deflation. Any dramatic changes in the quantity of money (deflation/inflation) will therefore have a “liquidity impact” on the economy. This paper, deals with the concept of liquidity. The “dual liquidity dilemma” in medieval Chinese society will be examined. This refers to the co-existence and the intertwining of deflation and inflation. It is argued that the dilemma was the consequence of the following factors: insufficient currency supplies, low value money repeatedly being minted, frequent policy change, and massive coin cutting. The dilemma was characterized with the pattern of “the alternative inflation disturbance in the long-term deflation.” This dual liquidity dilemma is supported by both textual resources and archeological evidence. This dilemma in medieval China was essentially caused by improper monetary policies in many political regimes. It was one of the major forces in shaping the socio-economic structure of this time period.