In this study, an efficient deep eutectic solvent (DES) catalytic system was developed by adjusting the basicity of hydrogen bond donors (HBDs) to synthesize high weight average molecular weight (Mw) poly (isosorbide) carbonate (PIC). It was shown that the Mw of PIC was made by the almost neutral value [EminOH] Cl-2EG was highest. The results of the transesterification kinetics with various catalysts confirmed that the rate of transesterification increased significantly as the basicity of DESs increased, but strong alkalinity inhibited the Mw of PIC. Therefore, the degradation of PIC was studied to uncover the mechanism of basicity at the molecular level by weight of the PIC. The strong alkalinity of DESs would cause the depolymerization of the macromolecular chains and ultimately limit the molecular weight of the product, proving the experimental fact that a near neutral catalyst is more conducive to the Mw of PIC. Finally, a reasonable mechanism of synergistic catalysis in combination with 1H NMR spectroscopy, Fourier infrared spectroscopy (FT-IR) and experimental results was proposed. The metal-free, inexpensive DESs catalyst with high catalytic activity used here is a practical candidate for the further development of polycarbonate on a biological basis.