In the PPP projects, the government guarantee provided by government department to private investors will become the government’s debt in the future, so relative to the direct funds guarantee way, the government departments like to be able to offer the guarantee which don"t need direct financial support, but in the face of many uncertainty the private sector hope to receive direct guarantee from the government department, in order to solve the above contradiction, this paper puts forward the government guarantee into concession period extended model, using the real option theory to calculate the government guarantee value, and then convert it to the specific concession period extended, finally using the actual case to test the mode, this way puts forward new ideas for the government guarantee mode.