This paper synthesis a new purifying agent on a rapid semidry process, using a natural guar gum as basic material, which was carboxymethyl guar. The factors which affect degree of substitution (DS) of carboxymethyl guar, such as the reaction temperature, the reaction time, the consumption of chloroacetic acid, and the consumption of sodium hydroxide etc were studied. The best synthesis condition was concluded. The flocculating property of the produced purifying agent was studied. Then the experiment analysis the structure and thermal stability through IR and DTA. The experiment results showed that the turbidity ratio of the micro-polluted water was better than other purifying agents. The synthetic purifying agent should have potential applications in the wastewater market.