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By using the Hubbard operator Green’s function method, the spin-resolved transport properties of a quantum dot coupled to metallic leads and side-coupled to a topological superconductor wire hosting Majorana bound states (MBSs) are studied theoretically. Compared with the second quantization representation, the method can lead us to an analytical result for the retarded Green’s function with finite U. The spin-related current and conductance are discussed. In the case of zero Coulomb interaction and on-resonance, the MBS’s 1/2 signature is recovered, and furthermore, there exists a 1/2 negative differential spin conductance. In the case of infinite Coulomb interaction, the 1/2 signature does not survive due to the Coulomb correlation reducing the current and conductance. Also due to this correlation, the MBS-induced symmetry of conductance peaks around zero energy is destroyed. In addition to this, we find that there are two MBS-induced negative differential spin conductance peaks. Theoretically, our work is supplementary and contrastive to the mainstream second quantization method, and these spin-resolved results may be observed in future experiments.