In 2012, rainstorm hit many countries and regions. New York, Beijing, Rome, Thailand and many other cities are affected in varying degrees, causing huge casualties and property losses It's urgent for us to make a design to provide assistance. According to reports, the main reason of causing massive hydrocephalus in cities is not drain sewage in time, and it is difficult for workers to find the manhole covers which are flooded. The manhole covers with buoyancy is an ingenious use of buoyancy of water to design. When the water level rises to a certain extent and the speed of water flow is over drainage speed that the manhole cover could control, the road cylinder hidden in the manhole cover will float. This kind of manhole cover could help workers find the manhole cover and drain water in time to minimize the loss. Besides, it could as a reminder that informs drivers and pedestrians that workers are working so that it will protect working personnel.