Abstract: The advantages of high efficiency and low fuel consumption of the contra-rotating propellers have attracted more and more attention. However, noise is the key problem that needs to be solved urgently in the development process of the contra-rotating propeller. Based on the nonlinear harmonic method and the theory of acoustic analogy, the influence of the number of rotor blades on the aerodynamic characteristics and noise of the contra-rotating propeller and its physical mechanism are studied. Taking a certain type of contra-rotating propeller as the reference object, seven kinds of contra-rotating propeller models with different combinations of blade numbers were studied. The study found that increasing the number of blades in the front/rear rotors of the contra-rotating propeller will increase the tension coefficient and power coefficient of the front/rear rotors, and the fluctuation amplitude will be significantly reduced, but it will also damage the total propulsion efficiency of the contra-rotating propeller. The paper proposes to increase the number of front/rear rotor blades while appropriately reducing the front/rear rotor speed. The results show that this scheme has two beneficial facets: On the one hand it ensure the total thrust and total efficiency of the counter-rotating propeller to be unaffected; on the other hand, under the interference noise frequency, it can obtain a more obvious and wider noise reduction effect than simply increasing the number of blades.