In later steam flooding period，steam breakthrough occurs easily. As a result，the oil production will decrease after the steam breakthrough. Serious steam breakthrough will lead to declination of oil-steam ratio and poorer development effect. In order to optimize the recovery factor and the economy of the development，the study of converting time in later steam flooding period is a very important issue. The decline history of heavy oil production in later steam flooding period was analyzed to understand the decline trend. A formula for oil recovery factor was obtained after characteristic formula of steam injection and production was derived. Finally the converting time was calculated by combining the equations.Through the analysis of certain well group of Qi40 block in Liaohe oilfield，a conclusion can be drawn that it is not suitable to convert to other development methods even the steam breakthrough occurs at the present period. The calculation result indicates that it is appropriate to convert steam flooding to steam-water alternate injection in January 2017. The simulation study also shows that the steam chamber can be fully developed in the reservoir till January 2017. More serious steam breakthrough will occur. So it is appropriate to convert steam flooding to other development methods. The calculation result agrees well with the numerical simulation results and it is important for the oilfield development guidance.