1. A method of obtaining a beam hardening correction coefficient for carrying out beam hardening correction on computed tomography data, comprising the steps of: acquiring an original reconstructed image and an original sinogram of an object of a particular size; obtaining an error-reduced sinogram after processing the original reconstructed image by error reduction; sampling and calculating an average value of the original sinogram and an average value of the error-reduced sinogram; optimizing the original sinogram according to the error-reduced sinogram to determine a coefficient vector of optimization function for the object of the particular size; and fitting the coefficient vector of the optimization function of the original sinogram to obtain the beam hardening correction coefficient for the object of the particular size.