Abstract: Due to natural and anthropogenic forcing, the ancient blue bricks in walls and dwellings of Pingyao ancient city have suffered a serious weathering diseases. They are in badly need of protection and restoration. This field investigation finds that all weathering diseases occur in the range of rising height of capillary water(1~3m higher than the ground surface).Main weathering types of the blue bricks include contour scaling, flaking, powdering, salt crystallization. Laboratory tests determines the mineral composition, major elements, physical properties, mechanical strength, as well as performance of resistance to salt crystallization and frost. The weathering reason of the ancient brick is the salts carried by capillary water when rises. The process is aggravated by frost also due to rising of the capillary water. Salts are derived from the soluble salts carried by capillary water when rises. Due to evaporation of capillary water and/or decrease in temperature, the soluble salts in the pores of the ancient bricks become crystal salt, in the meantime the capillary water is captured and transformed to crystal water. The process can generate powerful expansive pressure in the brick pores, leading to brick failure. Methods to prevent further weathering diseases are demonstrated. They are capillary water-proofing, salt-eliminating and brick-strengthening. Technical process and analysis method, presented by the paper on weathering mechanism and weathering prevention for the ancient brick, can provide references for protection and restoration of related buildings of cultural heritage.