At present, the main methods of analyzing the reasons why construction cost always got out of control are qualitative analysis based on the practical construction experiences in the construction cost management. And these analyses were easily affected by subjective factors. By introducing a complete decomposition model, the influencing factors of the incontrollable construction cost in the construction stage can be decomposed to the effect of quantity changes and the effect of unit price changes of each kind of construction. A quantitative analysis will be carried on subsequently to analyzing these effects to find the main effects of the whole construction cost changes. This model made the analysis more objective and scientific, and can accumulate the data for new or planned similar constructions. To illustrate, the decomposition on construction cost changes of a small civil project is carried out.