Abstract: The system reaction time of phased array radar is studied. The formula to compute the reaction time is proposed when TWS and TAS tracking mode are applied in phased array radar. The relationship between the reaction time and the searching, tracking and acquisition mode and their parameters are established. The optimization for the acquisition parameters is studied. The reaction times for the two tracking modes are compared. It is shown by the examples that the reaction time of TWS primly depends on the frame period of search and the target's velocity, and has nothing to do with the number of target. The reaction time of TAS mainly depends on the number and velocity of target, and is little effected by the frame period of searching.