Deep carbonate reservoirs are gradually becoming the main body for increasing reserves and production in China. However, due to the presence of fault zones in deep carbonate rocks, the reservoirs are fragmented and coring is difficult, resulting in limited research on their mechanical properties. This paper conducts a mechanical property analysis of the Ordovician Yingshan Formation carbonate rocks in the Shunbei block. Based on mineral composition analysis, the microstructure characteristics are studied. The hardness and elastic modulus of the rock samples are analyzed through nanoindentation experiments. Based on scratch tests, the strength weakening laws of different components (matrix, cementation surface, fillings, and fractures) in the rock samples under the circulation of the currently used drilling fluid are explored. The results show that under high-temperature and high-pressure conditions, the deep carbonate rocks exhibit significant strength weakening characteristics due to the action of the drilling fluid: as the soaking time increases, the strength of the rock sample matrix, fillings, and cementation surfaces continuously decreases, and the rock samples fracture along natural fractures. This study systematically clarifies the strength weakening laws of carbonate rocks under the action of drilling fluid, providing theoretical guidance for the calculation of drilling collapse pressure and the optimization of drilling fluid density.