The phase composition and microstructure evolution of pure and carbon-doped MgB2 bulks exposed to hydrochloric acid (pH=2) for different time were investigated by X-ray diffraction (XRD) and Scanning electron microscopy (SEM) systematically. XRD results show the decomposition of pure and carbon-doped MgB2 bulks exposed to acid is rapid decay with exposure time. The decomposition reaction will finish completely after 5 h exposure and the main solid products are B(OH)3 and MgCl2(6H2O). The SEM results demonstrate that the reaction among pure and carbon-doped MgB2 as well as HCl occurs at grain boundaries. But the grain connection of carbon-doped MgB2 is better than that of pure MgB2 after acid exposure. The Tc of MgB2 decreases after 10 min acid exposure, but Tc is almost unchanged in carbon-doped MgB2 after the same exposure. This result indicates that substitution of C for boron in MgB2 can improve of the resistance to the corrosion of acid