In the ISO9000“input- output”process model，the concept of process quality is difficult to get the correct interpretation. From the point of view of the process's“white box” theory，this paper puts forward the scientific meaning of the concept of process quality and the process quality model by taking the basic operating unit of6M1E in the railway construction project for example. The basic operating unit system consists of the technological natural process，operation process and management process；the process quality of the basic operating unit system depends on the interrelation and interaction among those three sub- processes，and also subject to the impact of the external disturbance input factors. Finally，the cast- in- situ prestressed concrete continuous box girder construction process is utilized to elaborate the specific application of this theory in the quality management of the railway construction project.