By means of precipitation, air temperature, snow cover, wind speed, and relative humidity influencing the construction, the Construction Workability Weather Index (CWI) is defined comprehensively. The construction weather conditions are classified in 5 levels according to CWI and their influences. With the data from 38 meteorological stations in Tibet from 1980 to 2013, the climatic conditions for construction workability in Tibet and its changes under the background of global climate change are analyzed. The results show that temperature and precipitation are the main factors that influence the meteorological grades for construction workability in Tibet; June, July, August and September are suitable periods for constructions; the number of suitable days for constructions appears to reduce from southeast to northwest; the start time of suitable periods delays from the southeast to the northwest; and the end time of suitable periods delays form the northwest to the southeast. At lower altitudes, there are more suitable days for construction, the sooner the construction period starts, and later the suitable construction period ends. With global climate warming, the number of suitable days for construction in Tibet increases, and the start time of suitable periods for constructions advances and the end time delays.