Abstract: In this paper,a floating underground garage,which connects with two multi rise residential buildings,was researched because concrete cracks or crushes occurred on the main members.Through monitoring and investigation on site,the reasons which caused the accidents and the failure mechanism were found that firstly,anti floating design was not carried out due to the wrong peak value of underground water level,which was less than bottom elevation of basement floor;secondly,because the progress of project wasn't completed on schedule,the benefit additional mass was not placed at the correct locations on time so as to make the damage of the members more seriously.Based on the results of monitoring and analysis,the combination of lowering the water level and adding additional mass was applied to deal with the incident,simultaneously,the damaged members were strengthened with RC and externally bonded steel plate.After the reinforced treatment was carried out,the performance of underground garage was good.The experience can be useful for similar projects.