The correctness of project decision-making is the premise of the rationality of project cost. Only when the correct and reasonable construction project decision-making can formulate an excellent investment and construction scheme, can the invested funds be scientifically allocated and managed, so as to make the project cost more reasonable. Moreover, in the actual implementation process, the project cost in the construction project can be effectively controlled. The main manifestation of the wrong construction project decision-making is that there is no comprehensive grasp of the content of the construction project, blind investment and construction, coupled with the unreasonable investment action plan, which eventually leads to the failure of the construction project investment, which not only causes the excessive consumption of resources, but also makes the construction unit bear huge economic losses, and even faces the situation of bankruptcy. Therefore, from this phenomenon, we can see that the control of project cost in the decision-making stage of construction project is very important. Only by effectively ensuring the correctness of construction project decision-making, can we ensure the accuracy of project cost budget and greatly reduce the occurrence of investment decision-making errors.