A method has been developed for the analysis of sodium distribution of oligonucleotides using matrix-assisted laser desorption ionization time-of-flight mass spectrometry（MALDI-TOF MS）.The single strand oligonucleotide primers（with a length of 20 bp，whose theoretical molecular weight is 6 071.02 Da） were dissolved in pure water and tested by traditional high performance liquid chromatogen-mass spectrometry（HPLC-MS） and MALDI-TOF MS respectively. The sample solution was tested by HPLC-MS and MALDI-TOF MS respectively after sodium acetate and sodium，alcohol precipitation and resolution，and the test mode was positive ion. Before and after sodium addition，HPLC-MS showed only the unsalted single component（corresponding molecular weight 6 069.9 Da and 6 069.8 Da） after deconvolution. MALDI-TOF MS showed a single component（corresponding molecular weight 6 071.09 Da） for the samples before salt addition，while the component distribution of different sodium number molecules was shown for the samples after salt addition：Each component started with no sodium molecules（corresponding molecular weight of about 6 071 Da），and its content showed a trend of first increasing and then decreasing，the number of sodium added ranges from 0-4 to 0-19，and the distribution changed with different salt adding conditions. The results showed that MALDI-TOF MS had the ability to detect the distribution of sodium salt components of oligonucleotides compared with LC-MS，and it also had a wide range of adaptability and sensitivity to detect a range of oligonucleotide molecules with different salt addition degrees. The method can also be used to analyze oligonucleotide sodium salt drugs（antisense oligonucleotide ASO，etc.）. In this paper，the antisense oligonucleotide drug Formivesen sodium was taken as the sample，and the pure product of Formivesen sodium and the animal injection model of Formivesen sodium containing polyethylene glycol and Tween were tested by this method. The results showed that the main sodium addition form of Formivesen sodium ranged from 0 sodium to 3 sodium，the overall content showed a decreasing trend，and did not change before and after adding the matrix. At the same time，the error of Formivesen sodium was also within the range of ±0.5 Da，and the accuracy was not affected by the matrix. In addition，several other components（including polyethylene glycol 300 and Tween-80） of the matrix were also shown independently in the mass spectrum，and there was no interference with the Formivesen sodium component. The method is simple，rapid，and highly sensitive. It is suitable for qualitative identification of a series of oligonucleotide sodium salt distribution，and the actual drug testing is not affected by the drug matrix，which is of great significance for the development and testing of antisense oligonucleotide（ASO） drugs.