The invention relates to a pre-concentrating process for high-temperature acidic gas, which includes the steps of: a) before thermal-regeneration of a sulfur-rich absorbent, performing effective heatexchange with a barren absorbent from a thermal-regeneration column, and after heat exchange, performing further heat exchange to the sulfur-rich absorbent via an over-heater so that temperature reaches 90-140 DEG C; b) performing flashing and gas stripping so as to further desorb the gases, such as CO2, in the sulfur-rich absorbent, thereby significantly increasing the concentration of H2S and achieving pre-concentration of the acidic gas at high temperature. Before entering the concentration step, the sulfur-rich absorbent is subjected to effective heat exchange via a first heat exchanger and a second heat exchanger with the barren absorbent from the thermal-regeneration column, so that energy is effectively utilized and production cost is reduced. In the process, bypass adjustment is also provided so as to effectively control the temperature of the sulfur-rich absorbent entering a flash column, thereby ensuring that the sulfur-rich absorbent is concentrated at proper temperature atdifferent degrees. Therefore, after the thermal-regeneration, Claus gases in different concentrations are produced in order to satisfy demands in downstream sulfur recovery work sections.