[en] The fission product poisoning of Xi'an pulse reactor is analysed deeply. At different power, the changes of reactivity of xenon and samarium before and after shutdown for Xi'an pulse reactor are calculated by using physical program. As well the results of calculations and measurements for Xenon poison and Iodine pit are compared. They are very similar, particularly, the difference of Iodine pit between them only is 5.7 x 10^-5. Owing to the reactivity of Iodine pit is very small, so Xi'an pulse reactor can be started any time after shutdown