The geometry plays an important influence on the performance of jet reactors. Based on the fast process of acid-base neutralization, the effects of geometric parameters such as the angle of diffuser, ratio of nozzle diameter to mixing chamber diameter and the position of nozzle on mixing and reaction characteristics were analyzed using CFD in detail. The results demonstrated that: (1) an optimum ratio of nozzle diameter to mixing chamber diameter was obtained, namely 0.35. If the ratio value is larger than 0.35, the conversion rate in the jet reactor decreases, whereas if the value is smaller than 0.35, the conversion rate can not improve any more but the pressure drop increases; (2) the larger the distance between the nozzle outlet and the inlet of mixing chamber, the faster the reaction proceed is. The optimum results can be acquired when the nozzle outlet shares the same plane with the centerline of the suction inlet; (3) the smaller the angle of diffuser section, the higher conversion rate in the same cross section can be achieved and at the same time, the bigger of the pressure drop is needed.