In allusion to the explosion of a urea reactor took place in a fertilizer plant at Pingyin, Shandong, China, a series of evidence collection and inspection jobs which includes collecting operation condition and parameters, sampling the explosion fracture, reactor body apart from explosion fracture, and leak detection medium and its hangover, etc., had been carried out firstly. Based on these jobs, farther analysis and computation work has been done to the structural and materials characteristics and the operation condition of the urea reactor, including compositions, metallographic phases, tensile properties, impact energy, strain ageing characteristics, and fracture toughness of the urea reactor steels, the compositions of leak detection medium and its hangover in the urea reactor, and explosion energy of the urea reactor. The most probable cause of the explosion of the Pingyin urea reactor may be: the severe stress corrosion cracking (SCC) and instable propagation occurred, which made the liner could not withstand the heavy load of internal pressure and produced tearing destruction, then caused the medium in the reactor leaking rapidly, and the BLEVE (boiling liquid expansion vapor explosion) happened in the reactor at last. The main medium reason which caused SCC is some minim alkali ions such as K+ and Na+ in the leak detection steam liquefied and concentrated. The structural reason to cause the explosive accident is the insufficient or uncertain seal of the conical thread connection between the leak detection nozzle and the first layer, which could not prevent the leak detection steam leaking and diffusing into the layer gaps. Finally, the leakage and concentration of the steam caused the SCC of many layers at the same time.