Abstract: The radioactivity calculation model in loss of coolant accident (LOCA) for AP1000 was established based on AST method and the change of radioactivity in containment and environment was studied after LOCA. The results show that the nuclide radioactivity in containment atmosphere first increases and then decreases with time, and reaches maximum at 40 min. According to the natural characteristics, the radionuclides can be divided into nuclides without nucleus decay and nuclides with nucleus decay. After 40 min of the accident, the activity of the former in containment exponentially decreases, such as ^131-135I and ⁸³Kr^m, while the activity of the latter decreases slowly due to nucleus decay, such as ⁸⁵Kr, ¹³³Xe^m, ¹³³Xe and ¹³⁵Xe. After several hours of LOCA, the cumulative release amount of I and Cs to environment increases slowly because of natural removal mechanism. For Kr and Xe, the cumulative release amount of the longer half-life nuclides to environment increases with time, while that of the shorter half-life nuclides reaches balance after a few hours of LOCA.