When the castable is mixed with water and cured at a lower temperature (less than 21 ℃), spalling occurs due to the formation of hydrated phase of calcium aluminate cement (CAC). CAH10 (C=CaO, A=A1203, H=H20) and alumina gel are the main bonding phases formed at this temperature. When the castables were heated to about 2112, CAHl0 dehydrated to form C2AH8 and AH3 gels. This gel usually reduces the permeability of castables and affects the drying of castables which are cured at low temperatures. However, the added organic fiber decomposes at a relatively low temperature (15012-20012) to produce a pore channel, so that the water vapor can be removed from the castable. Therefore, adding organic fiber can improve the air permeability and spalling resistance of castable. It has been proved that adding 0.05% polypropylene fiber can increase the apparent porosity and the average pore diameter, and increase the medium temperature strength of the castable.