Research conclusions：First of all, the project is decompounded on the first level framework, and the superposition of design and construction under the general contracting mode is analyzed from the angel of fast procurement. Then network diagrams under two modes are drawn. After comparing the Diagrams, it is found the project period can be shortened much by combining the design with construction well under the D-B mode. For instance, through calculation, the construction period of one project under D-B mode can be 92 days less than that under DBB mode, which is 15% of total construction period.