Taking the typical three-tube co-current ammonia synthesis reactor in a middle-size ammonia synthesis plant of our country as an example, the mathematical models of the reactor were employed to simulate the reactor operation calculation. The calculated sensitive parameters of inlet temperature, outlet temperature, inlet vacant velocity and outlet ammonia molar fraction of the catalyst bed and the daily ammonia output can be compared with those measured in industrial production to simply and quickly determine the catalyst active (coefficients) during different production periods,which has an important significance to instruct industrial (ammonia) production and ammonia synthesis reactor design.