Poly-aluminum-ferric-silicate-chloride (PAFSiC) coagulant was synthesized using coal gangue as raw material. The effect of silicon content on its chemical structure, coagulation performance and floc properties were systemically investigated. Results show that coal-gangue-derived PAFSiC consists of new species with silicon oxygen-metal bonds and impurities like NaAl(SiO3)(2) and K4Ca(SiO3)(3). The Si-O-Al and Si-O-Fe bonds were increased with the increase of silicon content. However, a certain amount of silicon ((Al + Fe)/Si molar ratio of 10:1-13:1) is required for the chain-net structure formation, too high or too low silicon content would lead to the destruction of this structure. The jar-test results showed that the best removal of color, UV absorbance at 254 nm and dissolved organic carbon was achieved at same (Al + Fe)/Si molar ratio (10:1-13:1). Moreover, the PAFSiCs with (Al + Fe)/Si molar ratio of 10:1 and 13:1 also demonstrated the highest floc strength and floc fractal dimension, which implying better settling ability in the subsequent sedimentation process. This study indicates that the introduction of silica into the hydrolyzing metal helps to increase the molecular size and enhance the bridge-aggregating ability of the coagulant. However, a proper silicon content is required for obtain the optimal coagulation performance and floc properties for coking wastewater treatment.