Abstract: The dissolution of sodium iodine ¹³¹I capsules for therapy was measured by the real-time online detection device of the dissolution of radiopharmaceuticals, and the dissolution curve was compared with the reference preparation. The results show that the in vitro dissolution behavior of self-made capsules are consistent with that of the reference preparation. It has been verified with good specificity of analysis method, good linearity (r>0.999) in the range of 1.9~108.4 μCi/mL of activity concentration, indicating that the method is accurate and reliable in which the recoveries are all in the range of 90%~110%, RSD<5%, with high precision.