Abstract: It is important to inverse the high accuracy real-time PWV by ground GPS for the short term prediction of bad weather.Currently the orbit and clock errors of satellites from ultra-rapid ephemeris are the main factors constraining the improvement of PWV's precision,the former usually affects the acquiring of PWV by differential GPS.17 GPS CORS sites from PBO network of America are used to test for estimating the ZTD by double differential mode with the tightly constrained initial precise coordinate as well as the correction of site displacements by combined ephemeris,the near real-time PWV was computed every 60 seconds.By comparing the result to the static mode,it shows high coincide between both the modes with most stations have only 2 mm differences or smaller,the inner precision can reach 1mm,the delay is less than 4 minutes and can meet the requirements of the real-time numerical weather prediction.The feasible region of PWV inversed by the ground GPS is acquired after analyzing the affection of baselines length on the above results,and also the affection of the heights difference on the PWV's accuracy the is discussed.