The invention discloses a process for preparing lactide through a fixed acid method, which comprises the following steps: (1) gradually heating a hydroxy or polyhydroxy or polyamino compound with a boiling point of 250-550 DEG C and lactic acid from 90 DEG C to 160 DEG C within 1.5-3 hours in the presence of a catalyst at a pressure controlled to be 2-3.0 KPa, then regulating the pressure to 0.5-0.8 KPa and the temperature to 170-180 DEG C, and continuing to react for 5-10.0 hours to obtain a hydroxy or polyhydroxy or polyamino lactic acid low polymer with a high boiling point; and (2) at the temperature of 180-230 DEG C and the pressure of 0.4-0.8 KPa, performing cracking reaction on the hydroxy or polyhydroxy or polyamino lactic acid low polymer with a high boiling point, which is obtained in the step (1), to obtain lactide. Compared with the lactide preparation method reported in public, the process disclosed by the invention achieves higher purity and better yield.