A new thermal-hydro-mechanical-chemical coupled acid fracturing model is proposed to analyze the effective acid penetration distance during acid fracturing. To improve the computational efficiency of the model, we used a simple three-dimensional displacement discontinuity method. Additionally, a finite-volume method was used to calculate the flow field. Temperature- and chemical-field models were also established based on energy- and material-conservation criteria, respectively. Subsequently, a new full implicit solution was established to solve the problem of rock deformation, stress interference, temperature, and acid–rock reaction coupling. The results show that the greater the injection rate, the greater is the effective acid penetration distance. To ensure that this distance is as large as possible, the injection speed should be maximized in the acid-fracturing process. Similarly, a high acid concentration results in a larger effective acid penetration distance. Therefore, the original concentration of the acid may be moderately increased, which is the simplest and most straightforward way to achieve a high acid penetration distance. If increasing the viscosity does not reduce the transfer rate of H+ and the order of the acid–rock reaction, the effective acid penetration distance is reduced. Therefore, the inter-dependence of multiple fields during construction optimization should be considered. In addition to construction and fluid parameters, reservoir parameters also have a significant effect on the effective acid penetration distance and acid-fracturing process. Reservoir temperature influences the effective acid penetration distance, fluid-temperature distribution in the fracture, and etching width. Finally, a high reservoir temperature results in a rather small effective acid penetration distance.