The invention discloses a urea crystallization detecting and testing method of an SCR tail gas treatment system of an engine. The method includes the steps of firstly, acquiring the weight M1 of an exhaust treatment device (5) before a test; secondly, in a testing cycle, allowing the engine to work and be tested under preset testing conditions, namely controlling the jet flow of a urea nozzle (4) to be a preset value or in a preset range A, controlling the exhaust flow of an exhaust pipe (2) of the engine to be a preset value or in a preset range B, and controlling the temperature in the exhaust treatment device to be a preset value or in a preset range C; thirdly, acquiring the weight M2 of the exhaust treatment device after the test, subtracting M1 from M2 to obtain the increased weight D of the exhaust treatment device in the testing cycle; fourthly, determining the urea crystal weight M in the exhaust treatment device under the preset testing conditions according to increased weight D of the exhaust treatment device. By the detecting and testing method, urea crystal amount can be tested and detected to serve as reference for judging adaptability of the SCR tail gas treatment system, and corresponding adjustment can be performed.