After Fukushima accident, the treatment of radioactive liquid waste has been highly concerned. The analysis of nuclide activity in containment liquid waste after severe accident is the premise of the liquid waste treatment. The source and reduction mechanism of nuclides in containment liquid waste after severe accident were analyzed. The calculation model for the nuclide radioactivity was established. The parameters of AP1000 were used for calculation on the radioactivity in containment liquid waste, and the radioactivity change of main nuclides in the containment liquid waste was studied. The results show that during the release of nuclides from core into containment, the radioactivity of each group nuclides in the containment liquid waste gradually increase, and then with the exception of the lanthanum group nuclides, the radioactivity of other group nuclides gradually decrease with time. The total iodine radioactivity decreases the fastest, and the total cesium radioactivity decreases the slowest. In the early stage of the severe accident, iodine is the main nuclide in the liquid waste, followed by the cesium group nuclides. The proportion of cesium activity in the total activity decreases first and then increases, while that of other groups increase first and then decrease. It is concluded that ¹³⁴Cs, ¹³⁷Cs, ⁸⁹Sr, ⁹⁰Sr, ¹²⁷Te^m, ¹⁰³Ru, ¹⁰⁶Ru, ¹⁴⁴Ce, ²⁴¹Pu, ⁹⁰Y, ⁹¹Y, ⁹⁵Nb and ⁹⁵Zr decay very slowly, which are the important components of the radioactive liquid waste.