In karst fracture-cavity reservoirs, the oil/gas is primarily distributed in natural fractures and caves. Therefore, hydraulic fracturing is frequently performed to connect the wells and natural karst caves to enhance the oil/gas production. In this work, the mechanisms of connecting the caves through hydraulic fracturing are examined through experimentation, numerical modeling, and field observations. The experimental and numerical results show that three main connection modes exist for natural caves, specifically connections through straight main fractures, crooked main fractures, and pre-existing fractures, corresponding to connection modes I, II, and III, respectively. In the experiments, 6%, 17%, and 77% caves were noted to be connected through connecting modes I, II, and III, respectively. Connection mode III was the most effective, exhibiting a maximum angle value of 225°. When the geostress difference is large, few natural fractures are active, and the connection mode I/II frequently occurs. When the geostress difference ≤ 4 MPa, injection rate ≤ 10 ml/min, fracturing fluid viscosity ≤ 10 mPa s, a great number of natural fractures are active, and the connection mode III frequently occurs. Therefore, preexisting fractures must be exploited to enhance the production of oil/gas in karst fracture-cavity reservoirs. Field observations indicated that by gradually injecting an acid fluid, the activated number of pre-existing fractures can be increased, thereby enhancing the efficiency of hydraulic fracturing and increasing the average daily oil/gas production. Keywords Fracture-cavity reservoir • Carbonate rock • Natural fracture • Hydraulic fracturing • Modes of cave connection List of symbols E Young's modulus of rock ng Total number of integration points in the computational area ν Poisson's ratio of rock ui(x) Local approximation of node i uh(x) Global approximation ωi(x) Weight functions of node i Computational domain associated with an element vis Visibility zone φi(x) Shape functions of traditional finite element method φi′(x) Subweight functions σ v Vertical geostress σ H Maximum horizontal geostress σ h Minimum horizontal geostress σ cri Critical maximum principle stress σ i,1 Maximum principal stress at integration point i σ j,1 Maximum principal stress at integration point j σ tip Stress tensor at fracture tip σ tip,1 Maximum principle stress at fracture tip