The invention relates to a method and a device thereof in the technical field of mobile communication technologies, in particular to a Log-MAP based decoding method and a decoding device thereof in turbo decoding. In the invention, in a Log-MAP decoding structure, the realizations of a forward state measurement and recursion structure and a backward state measurement and recursion structure are separated from the realization of an LLR computation structure; in LLR computation, an eight-input Logsum structure is not realized by adopting a two-input Logsum structure any more; the recursion of forward state measurement and the backward state measurement is realized by adopting a Max-Log-Map structure, so that the two-input Logsum structure still maintains a lowest realization complexity, meanwhile, the realization complexity of the eight-input Logsum structure is also reduced; and most importantly, the decoding properties and good properties close to an optimal Log-Map method are also maintained. The invention obtains good balance between the realization complexity and the properties of a decoder.