At moderate Reynolds numbers, airfoils are prone to generate intense tonal noise, leading to a significant increase in airfoil-radiated sound level. In order to have a deeper understanding on the generation mechanism of tonal noise, this paper investigates the generation and propagation features of the tonal noise waves radiated by a two-dimensional NACA0012 airfoil using Direct Noise Simulation method. The simulated Reynolds number is 2.31 × 105 in conjunction with an attack angle of two degree. The unsteady flow phenomena around the airfoil and the evolution process of sound waves both in the time and space domains are analyzed systematically. Through Proper Orthogonal Decomposition (POD), the dominant flow field modes at this angle of attack are extracted, and the contribution of primary flow structures to acoustic radiation is quantitatively evaluated. Results indicate that the unsteady flow structures near the trailing edge serve as the critical factor in tonal noise generation. The mechanistic insights derived from this study establish a foundation for developing targeted control strategies directed at dominant coherent flow structures, thereby enabling optimization of flow characteristics and suppression of noise generation and propagation.