Abstract: Waste drilling fluids in Changqing gas field are treated with gel-breaking and solid-liquid separation technologies to conform to the environment protection requirements. In gel-breaking the drilling fluids, a new gel-breaking agent, CQPJ-1, was developed to replace the single-purpose, slow and low-efficiency conventional gel-breaking agents. CQPJ-1 is able to reduce the quantity of electric charge of clay colloids and suspended particles, thereby decreasing the coagulation stability of the suspension system. By oxygenolysis, CQPJ-1 decomposes polymer materials, producing primary radicals and initiating a chain oxidation reaction, thereby dramatically reducing the molecular weight of polymer. CQXN-1, a cationic polymeric flocculant with moderate molecular weight, was selected to flocculate destabilized clay particles through its cationic group and long molecular chain, making the flocs much easier to mechanically dewater than flocs flocculated with inorganic flocculants. Laboratory evaluation has been conducted on the performance of gel-breaking agents, including H₂O₂, potassium permanganate, sodium phosphite and CQPJ-1, and the performance of flocculants such as poly aluminum chloride, poly ferric chloride, aluminum sulfate and CQXN-1. It was found that, using existing solids-control equipment, a waste drilling fluid with pH adjusted between 6.0-6.5, after treatment with 2%CQPJ-1 for 3 hours and then adding 0.15%CQXN-1, can be solid-liquid separated at 2,200 r/min. The separation efficiency and results have achieved the expected goal. Contamination index of the liquid phase from the treated waste drilling fluid was greatly reduced, and drilling fluids prepared with the liquid obtained had properties conforming to the requirements of drilling operation.