Abstract: Sandstone matrix acidizing is a very effective stimulation method to remove the damage nearby the wellbore in sandstone reservoir The study on acid/rock reaction and mechanism of secondary damages to formation is the basis of optimizing acid system and acidizing design. The Environmental Scanning Electron Microscope (ESEM) plays an importance role in the work. By saturating the specimen chamber with water vapor during examination, the ESEM allows wet or oily specimens to be imaged and analyed in their "native" state without drying, freeing, cleaning or coating with a conductive layer. Based on the features of ESEM and along with core acidizing, the change of mineral, porosity, and wormhole at the same position of the core can be imaged. The high magnification images provide detailed information about the change of core.