Colloidal particles has long been a challenge in wastewater treatment due to its small size and high surface charge, which inhibits the aggregation and sedimentation. Usually, a combination of inorganic and organic flocculants are used for efficient separation of colloidal particles from wastewater, resulting in high cost of water treatment, large production of solid slag and lengthy technological process. Currently, inorganic-organic composite flocculants are often synthesized through physical blending method, but the flocculation performance is not so desirable owing to the uneven mixing of inorganic and organic components. In order to solve the above problems, the inorganic-organic hybrid material poly(silicate aluminum chloride-acrylamide) [P(SAC-AM)] was synthesized through situ-polymerization of organic monomers in inorganic solution, and the characterization result suggested that inorganic particles were well distributed in organic matrix. The effect of P(SAC-AM) was investigated for flocculation of colloidal particles in coal gasification wastewater, and the commercial polysilicate aluminum chloride (PSAC), polyacrylamide (PAM) and polydimethyldiallyl ammonium chloride (PDMDAAC) were tested for comparison. The results indicated that P(SAC-AM) performed better at a lower dosage than commercial flocculants which were used either alone or in combination, and it was mainly owe to high-efficient synergistic effect of charge neutralization and adsorption bridging. Larger and dense flocs were formed by P(SAC-AM), thus facilitating the sedimentation and separation of colloidal particles. In addition, small amounts of COD, Ca2+, Mg2+ and other pollutants were also removed by P(SAC-AM) from wastewater. This result provides a facile and efficient way for separation of colloidal particles from wastewater, and it would be beneficial to shortening the technological process and reducing the cost of wastewater treatment.