We got the main reason of aridity from studying on the time-space distribution regulation of runoff of main rivers in Gansu province. That is, for all of the main rivers, annual runoff quantity,plentiful (rich)water period was very short ,generally ,only last 2～3 years; But low water period was very long may last 7～26 years. The average runoff remained steady basically for many years to many rivers in Qilian mountain regions; And we forecasted that the water quantity of all main rivers in whole province will change from low water years to more than normal water years from 1998～2009. The author suggested that everyone should establish a thought of anti-drought for long times.The utilization of water resources must be reasonable and optimal arrangements based on systematic analysis.