Abstract: False alarm problem of radioactive iodine activity monitor during commissioning in nuclear plant is analyzed and studied. Results show that the instrument background noise, ambient gamma radiation field and other non nuclide measurement statistics will cause the measured value distortion; the using of the ¹³¹I detecting efficiency to calculate the ¹³¹I to ¹³⁵I total radioactive iodine volume activity will cause the distortion of the measured results. Through the analysis of the reasonability of the set area of the radioactive iodine monitor energy window, and to change the measurement window energy 100~2000 ke V to 310~410 ke V, only ¹³¹I is monitored. After the implementation of this program, the instrumentation values are accurate and no false alarm occurs.