Abstract: In order to reduce reservoir damage during drilling and to improve temperature resistance, salt tolerance and foam stabilization of micro-foam drilling fluid, a new type of solid-free micro-foam system with temperature resistance and salt tolerance was prepared by synthesizing the foaming agents LF-2 and foam stabilizer HMC-1 and optimizing viscosity increasing agents and fluid loss additives. Laboratory performance evaluations showed that the cutting recovery rate was 88.7%, the recovery rate of core permeability was 88.87% and the circulation lost was only 1.8 mL in 40/80 mesh sand thief zones within 30 min. Later, the new system was used in underbalanced intervals drilling of Well Ci120. Results indicated that this new system was stable and adjustable, and drilling improvements were remarkable with average ROP 4.41 m/h and average hole enlargement rate 3.97%, respectively. In addition, the maximum total hydrocarbon of the intervals was up to 100% with success ignitions. In summary, the solid-free micro-foam drilling fluid system is strong in terms of temperature and salt resistance, and it demonstrates good performance in reservoir protection is worthy of further large scale application.