The precision of dimensional measurement plays an important role in guaranteeing the assembly accuracy of its internal components during the upgrading phase of EAST device. In this paper, the experimental research and analysis were done based on three dimensional combined measurement systems, combining Laser Tracker, flexible Measure ARM and measurement fiducials network, which are used for alignment and measurement of EAST components during the assembly process. The error sources were analyzed, e.g. temperature, gravity, welding, and so on. And the effective weight of each kind of error source was estimated by the simulation method. Then these results were used to correct and compensate the actual measured data, the stability and consistency of the measurement results was greatly improved in different measurement process, and the assembly precision of the EAST components was promised.