Abstract: Tight oil reservoirs in Ningdong Oilfield in Ordos Basin are characterized by complex lithology, low porosity and permeability, and strong heterogeneity. Drilling fluid losses occur frequently during the drilling process, and the invasion of the solid and liquid phase can easily cause damage to the formation. In order to determine the microscopic characteristics and damage mechanism of the tight oil reservoir in Ningdong Oilfield and reduce the damage to the reservoir during drilling and completion, systematic experimental studies such as scanning electron microscope (SEM) and solid-liquid damage tests were carried out. It was determined that the main causes of damage to the studied tight oil formation were solid phase invasion, water blockage, and associated water and salt sensitivity. The damage mechanism of the tight oil reservoir in this oilfield was clarified. In addition, it was found that the potassium ammonium-based polymer drilling fluid caused great damage to the tight oil formation, and the permeability recovery was low when the reservoir was treated with invisible acid. In view of the damage mechanism of tight oil reservoirs and the shortcomings of drilling fluid used in the oilfield, a low-damage and solid-free drilling fluid was constructed with a viscosity of 45.5 mPa·s, an API filtration loss of 3.5 mL, achieving a permeability recovery more than 85%, which caused low damage to the formation and could meet the requirements of protecting tight oil reservoirs in Ningdong Oilfield. The research results can provide a basis for formulating technical protection measures for tight oil reservoirs in Ordos Basin.