In entrained flow gasifier, slag properties such as melting and viscosity of molten slag are critical for continuous operation. Molten coal slag belongs to silicate-containing melts and slag behaviors are intrinsically determined by the microstructure transformation. Calcium is believed as an effective ingredient to decrease viscosity as well as reduce the polymerization degree. The structure of coal slags with different calcium ranging from 10% to 25% is investigated by solid-state nuclear magnetic resonance (SS-NMR) and molecular dynamics (MD). Si-29, Al-27 spectra were obtained through quenching slags under 1873 K and the distributions of Q(n )species are analyzed. For simulation part, the radial distribution function, the proportion of bridge oxygen (O-b) and non-bridge oxygen (O-nb) as well as the distributions of Q(n) species were calculated. Both NMR experiment and MD simulation show that increase of CaO can lead to a reduced degree of polymerization. The Ca ions enter into the network formed by Si and Al tetrahedral and bond with oxygen ions, which weakens the network stability. Consequently, the proportion of non-bridge oxygen increases and complex network unit-Q(4) breaks into smaller units such as Q(3), Q(2), Q(1), reducing the polymeric network. The depolymerization effect of CaO can explain the change of slag properties such as viscosity or melting temperature.