Abstract: The NW Yunnan area is one of the famous tectonic-metallogenic regions in southwestern China,and a series of engineering problems always occur because of the existence of altered rocks formed by hydrothermal action.It was shown by field investigation and lab tests that the types of altered rocks were mostly those of montmorillonitization,and the altered clay mineral was mainly pure montmorillanite,which rendered the altered rock both soft and expansive.The altered rocks had a similar development with the hydrothermal ore deposits in the area of three paralleled rivers.Taking examples from the NW Yunnan area along the Yunnan-Tibet Railway line,the geological characteristics and engineering properties of the altered rocks were discussed,and a series of control measures were put forward for the engineering problems caused by them.