This paper has derived a new molecular model for simulating semi-rigid main-chain LCPs (liquid crystalline polymers) based on the traditional GB (Gay-Berne)/LJ (Lennard-Jones) model, named Solo-LJ-SP-GB model. A single LJ unit and two nonlinear springs have been firstly used to describe the flexible spacers between GB units in the LCPs. Using the current model, the simulation time is less than 10% of that spent by using the traditional GB/LJ model, when simulating the semi-flexible main-chain LCP of 169 molecular chains with 6 flexible spacers between neighboring GB units, which greatly enhances the computational efficiency. Through using the current model to study the phase behaviors of semi-flexible main-chain LCPs, the odd-even effects of thermodynamic properties depending on the number of flexible spacers and the phase transition from isotropic to liquid crystalline have been observed, which agrees well with the existing experimental data and the simulation results of the traditional model. It is shown that the new model enables to describe the structural properties of semi-rigid main-chain LCPs accurately.