The comparative data of air pressure at manual and automatic observation at 97 weather stations in 2004, 2005, 2007 are used to analyze the difference between the two systems in Shaanxi and different areas of Shaanxi. The result indicates that daily average air pressure of manual observation is 021 hPa  higher than the automatic one and the standard deviation is 030 hPa. There is obvious difference among different areas. However,there is no obvious difference between DYYZ and CAWS instrument. Daily and monthly variety regulation is obvious. The time difference between automatic recording and manual observation has certain influence on hourly data but has no influence on climate analysis. The calibration value drifting is the main error for pressure sensor. The pressure data recorded by automatic stations can be continuously used with manual one.