1,4-butylene bi-p-hydroxybenzoate (BBHB) was synthesized using methyl p-hydroxyl- benzoate and 1,4-butanediol under catalyst and stabilizer conditions; Then, the thermotropic main- chain liquid crystalline polysiloxanes (MCLCPs) with flexible spacer, were synthesized using BBHB and dimethyldiethoxysilane reagents by melting polycondensation. The chemical structures and properties were characterized by FT-IR, polarized optical microscopy (POM) and differential scanning calorimeter (DSC) respectively. It was found that MCLCPs was the type of smectic A liquid crystal, and the liquid crystal temperature ranged from 58°C to 143°C, i.e. its variationrange reached 85°C. In addition, the polysiloxane was used as the flexible main-chain and the flexible chains CH2 was embedded into the rigid structural blocks of liquid crystalline polymers, which not only decreased the melting point below the thermal decomposition temperature, but also made the clearing point and a stable liquid crystal state observed.