In this paper, the effect of fly ash quality on the crack resistance was discussed. The lower the loss on ignition, alkali content and SO3 content of fly ash are, the better the crack resistance of the blended cement paste is. It is worthy to point out that properly increasing the fineness of fly ash is favorable for improving the crack resistance. Therefore the major quality indexes of fly ash are loss on ignition and water demand ratio, in the ?next place comes fineness. The alkali content of fly ash has disadvantage to the crack resistance of cement, so its quantity must be limited.