The acceleration mechanism of liquid alkalifree accelerating components, including aluminum sulfate and aluminum fluoride, is discussed by testing on the setting time of cement paste mixed with aluminum sulfate and aluminum fluoride respectively, as well as by XRD analysis on the cement paste mixed with accelerating components after final setting, 1day hydration and 7day hydration. The study results show that, due to the large amount of ettringite generated and the C3S hydration caused by calcium ion reduction and hydration heat, the setting of the cement paste is accelerated when aluminum sulfate solution is mixed into the Portland cement; due to the C3AH6 generated, the setting of the cement paste is accelerated by aluminum fluoride solution; the aluminum ion in the aluminum fluoride solution accelerates the hydration of the cement paste; the fluoride ion in the aluminum fluoride solution retards the setting of the cement paste. The paper can provide a theoretical basis for developing and promoting the application of highperformance liquid alkalifree accelerator.