The present invention discloses a SCR system automatic control method based on ammonia spraying sensitive valve locking. The method comprises three steps: firstly, performing comprehensive analysis of varying load condition operation characteristics of the SCR system based on a field test and a CFD numerical simulation technology to obtain flue gas 'NOx flow' regional continuous distribution characteristics matching an AIG type; then determining the weight of each ammonia spraying branch pipe valve, locking a certain number of sensitive valves according to a preset sensitive valve locking principle, and installing an automatic regulating valve at a corresponding ammonia spraying region; finally, developing an ammonia spraying logic controller and corresponding main control and sub-control logics, realizing multistage optimization setting of parameters of the control system, and realizing the accurate automatic control of the main ammonia spraying regulating valve of the SCR system and the locking sensitive valves. The SCR system automatic control method based on ammonia spraying sensitive valve locking is able to overcome the problems of characteristics of the SCR system such as nonlinearity, large delay and the like so as to realize the accurate automatic control of the ammonia spraying of the SCR system, substantially enhance the SCR denitration reaction efficiency, save the dosage of ammonia, reduce the ammonia escape and effectively inhibit the generation of ABS.