Produced water from gas fields in the eastern Ordos Basin is featured with high salinity, high hardness, and enriched iron ions. Abnormal rheological properties and out-of-control filtration loss are easily caused when the water is directly reused for composite-salt drilling fluid preparation. Elevated operational costs and limited valorization pathways for treatment by-products exist in conventional processes.To solve the above problems, a research protocol integrating single-factor tests and orthogonal experiments was employed. Key ionic components limiting the performance of composite-salt drilling fluids were clarified. Concentration thresholds of makeup water adapted to this system were defined. A three-stage treatment train composed of NaClO pre-oxidation, CO2-induced softening and ceramic membrane separation was established.Satisfactory purification performance is achieved. No detectable SS or total iron is found in the final effluent. The concentrations of Ca2+ and Mg2+ are kept within the threshold ranges. For drilling fluids formulated with treated water samples of low, medium and high salinity, the YP/PV reaches 0.471, 0.456 and 0.454 Pa·mPa?¹·s?¹ respectively. All performance indicators satisfy the demands of on-site construction. When the CaCO3 by-product derived from the CO2-induced softening procedure is adopted to fully replace the ZDS weighting agent, a YP/PV of 0.414 Pa·mPa?¹·s?¹ and a filter cake adhesion coefficient of 0.207 are obtained. A technical chain of "treatment-reuse-resource valorization" is constructed in this work. Technical support is offered for in-situ recycling and resource conversion of produced water in water-deficient gas fields.