A rapid urea and byproduct deposit experimental platform was built and an integrated after-treatment device model of the nonroad agricultural diesel engine was established based on CONVERGE software. The effects of exhaust temperature, exhaust flow rate, and urea injection mass on urea and byproduct deposits were studied by both experiments and simulation. The model was validated using the back pressures of catalysts, mixer wall temperature, and distribution of urea and byproduct deposits. The performance of the mixer was evaluated using the concentration, mass, and uniformity of NH 3 at the front-end cross-section of its selective catalytic reduction catalyst. Wall temperature is the key influence factor on the composition of the liquid film and is affected by the urea injection mass and exhaust flow rate. When the wall temperature is low, the liquid film is mainly composed of urea solids, but when it is above 423 K, urea solids begin to decompose into biuret, cyanuric acid, ammelide, and other byproducts. As for mixer performance, the uniformities of NH 3 , temperature, and velocity are above 0.9, 0.975, and 0.965, respectively, under all working conditions.