Abstract: Reactor core melt down accident may result in lower head failure, and molten material may relocate into the cavity, which endangers the safety of personnel and hull. The severe accident integration calculation program called MAAP4 code is adopted to study the effects of low pressure injection systems launching time and flow rate when the corium pool had formed for In-Vessel retention. The results show that two low pressure injection pumps can effectively prevent the failure of pressure vessels and the corium melt can be retained within the reactor when the corium pool had been formed for 1576 s, the low pressure injection systems cannot prevent the pressure vessel failure 2646 s after the corium pool had been formed.