Abstract: The Ordovician carbonate reservoir in Tarim oilfield is dominated by fractures and solution pores and cavities and characterized by ultra?deep, high pressure and high temperature, of which the most fractures with medium?high angle are as both reservoir spaces and the main flow channels. But since such a reservoir is of low pore?throat coordination and poor connectivity, acid fracturing process has to be needed to build its productivity. The main factors affecting the effect of acid fracturing include the selection of acid fracturing process for the long openhole interval and that the acid?etching fractures can communicate the effective fracture?cave reservoir or not. For these reasons, this paper conducts such parameters optimizations as acid?etching fracture length, fluid conductivity, stimulation scale, prepad fluid ratio and expelling amount, etc., and then selects the fracturing and acid liquids to form a differentiated and multistage acid fracturing optimization design scheme. The field application indicates that the designed parameters are reasonable and effective, and the multi?stage acid fracturing technology is suitable for stimulation of the ultra?deep fractured carbonate reservoirs