This study investigated the relation between seasonal stratification and sediment phosphorus release in a seasonally stratified lake. Lake quality monitoring was conducted for 10 months from April 2010 to January 2011 in Lake Hongfeng, southwestern China. Destratification generated strong mixing from surface to bottom waters in the early autumn. The lake water was well-mixed vertically within 3 days and became anoxic within next few months. Total phosphorus concentrations of the whole water column increased by 15–20% as a result of high surface sediment concentrations of total phosphorus and reactive phosphorus. In seasonally stratified lakes, high reactive phosphorus content in sediments may form a larger flux of dissolved phosphorus in the sediment–water interface following turnover. Technical measures should be taken to increase dissolved oxygen concentration and control the water circulation in this lake, especially during the cooling period, to weaken the effect of destratification and prevent sediment phosphorus release.