Abstract: A method of producing linear polyethylenimine involving mass polymerization of ethylenimine or its oligomers: dimer, trimer or linear tetramer taken individually or in a mixture at a temperature within the range of from 40.degree. to 55.degree. C in the presence of cationic catalysts such as mineral acids, ammonium salts of mineral acids: hydrochloric acid, sulphuric acid or chloric acid, ammonium salts of hydrochloric or chloric acid, dichloropropane or dichloropropanol taken in an amount ranging from 0.1 to 1.0 mol.% per each elementary unit. Said polymerization in mass is performed till a conversion ratio of ethylenimine cycles of 10 to 25% is reached. Thereafter, the resulting polymerization mixture is cooled to a temperature within the range of from 5.degree. to 25.degree. C, added with water at a weight ratio between water and the polymerization mixture ranging from 1:2.5 to 2.5:1 and the mixture is maintained at a temperature within the range of from 5.degree. to 25.degree.