Solid content of submicron particles in drilling fluid has a great influence on the performance of drilling fluid. It is necessary to research targeted and effective flocculant to remove submicron poor solid and minimize the flocculation of useful solid such as bentonite. Using kaoline and bentonite as flocculation objects，the flocculation effect of the long chain alkyl quaternary ammonium salt，dimethyl distearylammonium chloride（DODMAC），and the flocculation mechanism were investigated by the zeta potential，turbidity analysis，particle size analysis and potential energy calculation. The results showed that the kaoline dispersion reached the point of zero charge near the pH 5，and its surface electrical behavior and electric quantity were susceptible to pH，while that of bentonite was more stable. When the dosage of DODMAC was 42.86 mg/L，turbidity removal rate（TRR）of the kaoline dispersion was more than 90%，but it gradually decreased when the dosage was more than 42.86 mg/L. Meanwhile，the TRR of the bentonite dispersion gradually increased with increasing the dosage of DODMAC，moreover，the TRR was very low compared to that of kaoline dispersion. Therefore，the selective flocculation of kaoline could be achieved by adjusting the amount of DODMAC. DODMAC could reduce the energy barrier needed to overcome when kaoline occurred agglomeration and promote the agglomeration degree of kaoline particles through electric neutralization and hydrophobic flocculation. When the dosage of DODMAC was too large，the surface electrical behavior of kaoline particles would be reversed and the zeta potential would increase，which resulted in that the electrostatic repulsion between the kaoline particles increased and kaoline particles dispersed. As for the bentonite dispersion，DODMAC played an adsorption-bridging role in flocculation of the bentonite particles.