Abstract: The IMO Tier Ⅲ regulation sets strict requirements for NO_X emissions of marine diesel engines, and Selective Catalytic Reduction (SCR) serves as the core technology for emission compliance. Scheme B is an important emission approval approach for SCR systems. Nevertheless, ammonia is adopted as the reducing agent in model tests, which cannot reproduce the pyrolysis and hydrolysis processes of urea-water solution. It is therefore difficult to directly obtain urea demand under the E3 cycle condition, bringing challenges to injection strategy calibration and on-board approval. To solve this problem, a marine diesel engine equipped with the SCR system was taken as the research object. SCR reactor model tests and three-dimensional simulation analysis were jointly carried out to obtain the required Ammonia-to-NO_X Ratio (ANR) under various load conditions, and a correction scheme for urea pyrolysis and hydrolysis was established. The urea-water solution demands under 100%, 75%, 50% and 25% loads of E3 cycle are 38.8 L/h, 36.7 L/h, 28.3 L/h and 17.0 L/h respectively. The on board test shows that the maximum relative error of NO_X conversion efficiency between measured data and model-test results is 2.34%. This method can determine the demand of urea-water solution for emission approval of Scheme B, and provides a feasible technical way to formulate urea injection strategies for Selective Catalytic Reduction systems of similar marine diesel engines.