The invention discloses an alkali-free chlorine-free high-early-strength liquid accelerator and a preparation method thereof, and belongs to the technical field of building materials. The alkali-free chlorine-free high-early-strength liquid accelerator is synthesized by aluminum sulfate (a), alkaline earth metal salt (b), aluminum hydroxide (c), hydroxy carboxylic acid (d), an organic amine compound (e) and a stabilizing agent (f). The invention is characterized in that: the organic amine compound, a polyhydroxy organic matter and inorganic salt are subjected to complex reaction, and the molecular structure of the product is regulated by regulating a ratio of (a) to (b) to (c) to (d) to (e) to (f) so as to improve the performance of the product. Mortar doped with the product has the advantages of adjustable setting time, one-day compressive strength of over 20MPa and a 28-day compressive strength ratio of about 100 percent, the synthesis process is simple, reaction conditions are mild, and the alkali-free liquid accelerator has good performance.