The gear structure is equivalent to disposed as a circinal plate, and also, reference circle cracks of gear are simulated by setting the arc direction crack. Because of the appearing of crack, it will divide the whole circinal plate into some areas, and set different polynomials of vibrational flexibility in every area, then using District Technology and Rayleigh-Ritz method, the dynamical characteristic of circinal plate with arc direction crack is analyzed, the influence of the type and size of crack on dynamical characteristic(natural frequency, modal shape etc.)of circinal plate are deep studied. On this basis, making use of Finite element method, the first some order mode of the acoustic character of cracked plate are investigated using discrete Rayleigh integral, and the distribution of sound pressure on the r=0.5m sphere surface is obtained. At the same time, the influence of different depth of crack on acoustic characteristic of gear disc are discussed, and the rule of influence of crack on radiated acoustic field is given out. This result shows that the influence of crack on radiated acoustic field of circinal plate with the increase crack depth is increscent. These provide a theoretic foundation for gear fault diagnosis using the dynamic and acoustic method in engineering.