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In a selective catalytic reduction (SCR) system for large engines, a quicker evaporation of the urea-water solution (UWS) spray can be achieved through injection with air-assistance. It makes the SCR system more compact, which saves space required for its installation and the cost of manufacturing. A validated CFD study for the air-assisted spray breakup of UWS is not available in the existing literature. In the current work, the Lagrangian-particle-tracking (LPT) model along with the blob approach is used to develop a 3D-CFD simulation method, which can predict the air-assisted spray breakup of UWS in SCR systems. The method can predict sauter mean diameter (SMD), arithmetic mean diameter (AMD) and velocity of droplets. Simulations results are in good agreement with phase Doppler anemometry (PDA) measurements carried out at a hot gas test-rig. The method can also predict the influence of operating conditions on the spray characteristics and helps to determine an optimum mass flow rate of compressed air for the spray breakup process in order to get a desired spray angle and droplets sizes. The aim of the study is to develop a 3D-CFD simulation method for prediction of the air-assisted spray breakup of UWS for SCR applications.