We applied our recently proposed systematic and simulation-free strategy for the structure-based coarse graining of multicomponent polymeric systems (Wang, Q., Polymer 2017, 117, 315) to diblock copolymer melts, where we use the well-developed polymer reference interaction site model (PRISM) theory, instead of the commonly used many-chain molecular simulations, to obtain the structural and thermodynamic properties of both the original and coarse-grained (CG) systems, and to quantitatively examine how the effective non-bonded pair potentials between CG segments and the thermodynamic properties of CG systems vary with the coarse-graining level. We proved that our strategy does not change the spinodal curve, regardless of the original model system, closures, and coarse-graining levels for the two blocks. Using a simple original model system for symmetric diblock copolymer melts, we coarse-grained each block as N/2 segments and examined CG systems with N ranging from 2 to 100. We found that coarse graining (i.e., decreasing N) increases both the peak value and peak location of the partial structure factor characterizing the composition fluctuations in the CG system. Contrary to the common practice in the literature, CG potentials obtained from short-chain systems cannot be directly used for long-chain systems; this is in fact the transferability problem in coarse graining (Louis, A. A., J. Phys.: Condens. Matter 2002, 14, 9187). We also found that the structure-based coarse graining cannot give thermodynamic consistency at any coarse-graining level, consistent with our previous work on homopolymer melts (Yang, D. L.; Wang, Q., J. Chem. Phys. 2015, 142, 054905; Wang, Q.; Yang, D. L., Polymer 2017, 111, 103) and binary polymer blends (Wang, Q., Polymer 2017, 117, 315).