Aim The principle and the selection of parameter of the method of P-M diffusion equation for image denoising is analyzed.Methods The methods of fourier transform and orthogonal coordination based on the directions of the image feature are used.Results The discussed denoising principle based on these methods is more easily to understood than that based on P-M diffusion equation.Conclusion The method provides theoretical basis for the selection of parameter for P-M diffusion equation for image denoising.