The invention discloses a control method for ammonia spraying amount of a denitration device and a control device of the control method for the ammonia spraying amount of the denitration device. According to the control method, the difference value of the actual denitration efficiency value and a denitrification efficiency set value is compared with a first preset value to judge whether the ammonia spraying mass flow rate should be adjusted or not, if yes, the difference value of the actual denitration efficiency value and the denitrification efficiency set value is input to a PI controller to obtain a corrected value of the ammonia spraying mass flow rate, then the corrected value of the ammonia spraying mass flow rate is added to a reference value of the ammonia spraying mass flow rate to obtain a set value of the spraying ammonia mass flow rate, the difference value of the set value of the spraying ammonia mass flow rate and a measured value of the actual ammonia spraying mass flow rate is input to the PID controller to obtain a control instruction of an ammonia spraying control valve to control the ammonia spraying control valve so as to adjust the ammonia spraying mass flow rate. The control method for the ammonia spraying amount of the denitration device disclosed by the invention is simple, adjusted accurately, capable of ensuring the denitration efficiency to reach an expected value, and further can make the ammonia escape amount reduced below 3ppm.