Abstract: The main reservoir types in Ordovician carbonate formation in Tahe oil field are dissolution fractures and caves. Acid fracturing is one of the main methods of oil field development. Venting and leakage during sidetracking show that a lot of caves distributed nearby, but not connected to well because of single acid fracture extending direction. Increasing the complexity of acid fracturing may contribute to improve the probability of link between well and caves which distributed along different directions. A fracture extending compute model is proposed under the conditions of complex natural fracture. Different rock mechanic parameters which have influence on fracture's complexity are assessed:friction coefficient > horizontal stress difference > approximation angle > tensile strength > cohesion. Acid fracturing for complex fractures is applied in 5 wells, obtaining 5 909.7 t oil in average for each well.