The gear structure is equivalent to disposed as a circinal plate, and also, the root crack of gear are simulated by setting the radial 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 radial direction crack is analyzed, the influence of the 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 the 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; moreover, with the increase in crack distance from the fixed end, the crack of plate increases the effect of the sound field. So, these provide a theoretic foundation for gear fault diagnosis using the dynamic and acoustic method in engineering.