The invention relates to a propeller installing and positioning device and a propeller underwater installing method. The propeller installing and positioning device comprises a base support corresponding to a propeller so as to bear the propeller, a first float bowl and a second float bowl, wherein the first float bowl and the second float bowl are respectively connected with two sides of the base support. The first float bowl is provided with a first valve for injecting water into the first float bowl or discharging the water in the first float bowl and a second valve for injecting air into the first float bowl or discharging the air in the first float bowl, the second float bowl is provided with a first valve for injecting water into the second float bowl or discharging the water in the second float bowl and a second valve for injecting air into the second float bowl or discharging the air in the second float bowl, the second valves are connected with an air compressor through pipes, and air discharge valves and air control valves are arranged on the pipes. The propeller underwater installing method utilizes the propeller installing and positioning device to bear the propeller to an underwater specified place, the installing difficulty is reduced, the safety is improved, and a platform building period is shortened. In addition, the installing accuracy is high, and the installing cost is low.