Abstract: Abstract: The harvest of algal cells is difficult since the algal biomass density is usually less than 1 g/L and the cells have a small size (3-30 μm). At present, harvesting of microalgal is done with widely used techniques including coagulation-flocculation and sedimentation. Metal salts and macromolecule polymers (polyferric chloride and polyaluminum chloride) are common chemical flocculants for harvesting microalgae, but these metallic compounds may cause pollution. Therefore, it is necessary to develop efficient and non-polluting flocculants. Chitosan is a kind of non-toxic biodegradable polymer with good adsorption properties and important functions including electrical neutralization, bridging, and netting. At present, chitosan has been extensively used as effective flocculant, fungicide and ion exchanger for water treatment. It is widely used in food, agricultural, pharmaceutical, and chemical industries. Chitosan can be used as an environment-friendly, healthy, and safe flocculant for microalgae. However, chitosan is only soluble in acids, thus limiting its application in the flocculation process. Therefore, it is necessary to modify chitosan or adopt a composite material of chitosan with inorganic materials for the purpose of harvesting algae. Inorganic materials like metal salts may cause pollution. Clay minerals, cheap and conveniently available natural pollution-free materials, have been applied for algal flocculation. This study aims to develop non-toxic and efficient flocculant for harvesting microalgae. Three different types of clay minerals, i.e. bentonite, diatomite and zeolite, were modified with acidic chitosan to prepare 3 new types of composite flocculants, named CMD, CMB and CMZ, respectively. The flocculation efficiency (FE) was compared by flocculant concentrations, sedimentation time, pH values and the ratio of chitosan to clay minerals. The results indicated that the FE of modified flocculants was all obviously higher than chitosan for harvesting Chlorella pyrenoidosa. Among them, CMD flocculant had the best FE (95%) with a flocculant dosage of 0.2 g/L. Chitosan can change the surface electrical properties of diatomite in a way that the surface of diatomite becomes positively charged. The surfaces of microalgae are often negatively charged. This way has caused the FE of C. pyrenoidosa to be greatly improved. Moreover, the FE reaches the maximum of 93% with the ratios of 1:6 (chitosan:diatomite). When the ratio was changed to 1:14, the FE decreased by 10% because excessive diatomite was not mixed with chitosan. The best FE of C. pyrenoidosa was 94% at pH value of 8 and 91% at pH value of 9 with a dosage of 0.2 g CMD after 120 min sedimentation. The FE was about 28% higher than the values at pH value of 4 or 6. When the pH value was increased to 11, the FE was decreased by 17%. Therefore, the optimum pH value for the flocculation of C. pyrenoidosa was set as 8. In a word, the best FE reached 96.16% under the CMD flocculant dosage of 0.2 g/L and the pH value of 8 after 120 min sedimentation. In this study, the raw materials are non-toxic and without secondary separation for harvesting algae from water, which can be further utilized or processed for wide application.