Different kinds of mix proportion of concrete, e.g. sand ratio and sand mix, were designed. By testing the concrete slump flow, slump-flow time (T500) and combining with the calculation of paste film thickness (PFT) and water film thickness (WFT), the relationships between PFT, WFT and fluidity of fresh concrete were analyzed in detail. The results show that the fluidity of concrete is greatly effected by PFT and WFT, and the fluidity, PFT and WFT of concrete prepared with different kinds of sand are quite different. The slump flow of concrete increases with the increase of PFT and WFT, and the correlation is high between the fluidity and, and WFT. The T500 decreases with the increase of PFT and WFT, but the correlation is poor between T500 and PFT, WFT. The changes of PFT and WFT are mainly due to the changes of the fine content, fineness modulus of machine-made and aggregate porosity. When the fine content of sand is too high, although more slurry will be introduced to improve PFT, it will also reduce the volume water binder ratio and improve the specific surface area, resulting in the decrease of WFT and the deterioration of concrete fluidity.