1. An adaptive-finite-element-based multi-spectral reconstruction method, comprising: performing the adaptive-finite-element-based multi-spectral reconstruction by a processor by utilizing a posteriori permissible source region selection strategy according to a priori knowledge about multi-spectral information of a bioluminescent source and an anatomical structure and optical parameters of a given domain to be reconstructed, wherein the method comprises the following steps: Step 1: on the (l−1) th (l>1) mesh level of the given domain to be reconstructed, a single-spectral diffusion equation is discretized into a linear equation based on the finite element method; Step 2: based on the selection of a posteriori permissible source region, linear relations between unknown source variables and boundary measured data are established on respective single-spectral bands, and the relations on the respective single-spectral bands are combined into a uniform equation reflecting the multi-spectral bands by using the spectral characteristic of the bioluminescent source; Step 3: an optimization objective function is defined according to the regularization method, and then the spectral projected gradient-based large-scale optimization algorithm is used to optimize the objective function so as to obtain reconstructed results of the (l−1) th level; Step 4: a single-spectral surface light density distribution is calculated by employing the aforementioned reconstructed results, and it is determined whether the reconstruction process shall be terminated or not according to stopping criteria, wherein if any one of the stopping criteria is satisfied, the reconstruction process is terminated; Step 5: otherwise, if none of the criteria is satisfied, the permissible source region is reselected according to a posteriori error estimations, and the permissible and forbidden source regions are refined adaptively; Step 6: the (l−1) th level is transformed into the l th level, the parameters are adjusted, and then the method returns to the Step 1 to continue the reconstruction process.