A blade assembly, a power head and a lawn mower are provided, the blade assembly includes a blade base, a first blade and a second blade. A first boss and a second boss are arranged on a blade assembly surface of the blade base. A height of the first boss is a first height, and a height of the second boss is a second height. The first blade is provided with a first mounting hole matched with the first boss, the second blade is provided with a second mounting hole matched with the second boss. The first blade is mounted on the first boss, and the second blade is mounted on the second boss. Through arranging a step-shaped boss to position and mount a blade on the blade base, it is more convenient to realize an assembly and replacement of double blades.