With the development of construction industry, casting reinforced concrete floor has replaced pre-cast concrete hollow block floor with pre-stress. A large number of irregular cracks appear on the floor, which has become the biggest problem related to the residential projects’ quality. This paper analyses the causes of cracks and the major factors of the pre-mix concrete components and relevant material performances. Combined with current concrete technology, supply of raw material and production, and construction of concrete realities, controlling cracks can be begun to consider from two aspects, namely increasing the concrete strength and decreasing the concrete shrinkage. This paper puts forward the following measures to control cracks: 1. strict control of the water for concrete; 2.strict control of concrete slumps; 3. improving the quality of aggregate (such as sands or stones) and increasing the number of coarse aggregate; 4. Reasonable choice for additive; 5.controlling the amount of adulterant in concrete.