The invention discloses a free-cutting steel for automobile synchronizer, which comprises: C: 0.06% to 0.12%, Mn: 1.00% to 1.30%, Si: ≤0.08%, Cr: 0.10% to 0.40% by weight %, P: 0.050% ~ 0.100%, S: 0.32% ~ 0.40%, N: 0.006% ~ 0.015%, Al: 0 ~ 0.040%, [O] ≤ 0.0020%, the balance is iron and unavoidable impurity elements , and Mn / S≥3. The invention also discloses a preparation method of the steel for the free-cutting automobile synchronizer. Through the optimized design of the composition, the manufactured synchronizer has small deformation after heat treatment, good fatigue performance and stable mechanical properties. In addition, each process is strictly controlled according to the target composition to ensure the precise composition of the steel.