Acid treatment is an important measure to improve production for horizontal wells of carbonate reservoirs. Acid injection profile of horizontal wells (AIPHWs) is the most critical indicator of the success of the acidification. Currently, most previous works studied AIPHW based on the point source method. However, an inherent singularity exists in the point source solution, and hence, the calculation speed is too slow or the solution does not converge. To solve this problem, a semi-analytical model (coupling the wellbore flow model and reservoir seepage model) by the volumetric source method is presented to determine AIPHW in carbonate reservoirs. In this new coupled model, the permeability heterogeneity, formation contamination, acid-induced wormhole formation, and wellbore pressure drop are all considered. The results of the validations show that the results from the proposed method match well with the model results from the literature. Then, the effects of the acid injection flowrate, acid injection time, and permeability heterogeneity on the AIPHW are studied. According to the sensitivity analysis, we find that the disequilibrium degree of AIPHW becomes stronger as acid injection flowrate increases, acid injection time increases, and permeability heterogeneity coefficient increases. This study provides a guide for the design of horizontal well acidification and the evaluation of acid treatments of horizontal wells in carbonate reservoirs.