The effect of several types of fibers on the expansion of cement paste, in which a large amount of fly ash with high content of f CaO (HFA) was added, was investigated. A new idea to prevent cement paste with HFA from destruction was proposed. The experimental results indicate that different types of fibers show significantly different effects on the restraint of expansion. Carbon fiber and alkali resistant glass fiber which has high elastic modulus could effectively restrain the expansion. To our surprise, the expansion of cement paste with Nylon fibers of low elastic modules is greater than that of control group. Moreover, it was also discovered that thin fibers could prevent the pastes from cracking caused by instability of HFA effectively through testing their water boiled soundness of the mixed paste.