1. An attenuation correction method for an image in a Positron Emission computed Tomography (PET) system, comprising: acquiring transmission scanning sinogram data from a PET apparatus; reconstructing the transmission scanning sinogram data with Bayesian model-based Ordered Subset Expectation Maximization (OSEM-B) algorithm and Filtered Back Projection (FBP) algorithm, to obtain an OSEM-B attenuation image and a first FBI) attenuation image respectively; performing a weighted calculation on the OSEM-B attenuation image and the first FBP attenuation image to obtain an effective attenuation image; and performing attenuation correction on emission scanning sinogram data from the PET apparatus by using an attenuation sinogram generated based on the effective attenuation image.