Abstract: I-131 is an important artificial radionuclide and an important detection object in nuclear facilities. The current mainstream method is to use a uniformly distributed Ba-133 source instead of an iodine source to calibrate the I-131 detection instrument. However, in the process of value transmission, the distribution of the I-131 calibration standard source directly affects the setting value of the standard source and affects the calibration of the testing instrument. The plane source is used to determine the detection efficiency at different depths, and the depth efficiency curve is established. The fitting result is η(x)=0.007 655e^{-0.020 6x}. Therefore, the activity efficiency equation of I-131 was established through mathematical analysis, and the positive and negative measurement results were used to solve it to determine its activity distribution. The distribution of the three I-131 sources is uniform distribution, exponential distribution and plane distribution. The deviation of exponential distribution source is between the deviation of the positive and negative measurement results of the uniform distribution source，which does not exceed 1%，and the deviation of the positive and negative measurement results of the planar distribution source，which exceeds 50%. By judging the quick solution of the source distribution, it is easy to prepare the calibration standard source and establish the on-site calibration process.