The invention discloses a system and method for treating bamboo reed and modified biological carbon. The system comprises a pretreatment device and a revolving bed pyrolyzing furnace, wherein the pretreatment device is provided with a bamboo reed inlet and a pretreated bamboo reed outlet; the revolving bed pyrolyzing furnace comprises an annular furnace chamber, a rotary material distributing plate, a hot air inlet, a material inlet, a pyrolysis oil gas outlet, a biological carbon outlet, a grinding device and a mixing tank. A smelting space is defined by the annular furnace chamber; the hot air inlet is formed in a heat supply chamber; the material inlet is formed in a pyrolysis chamber and connected with the pretreated bamboo reed outlet, and a material mixer is arranged at the material inlet; the pyrolysis oil gas outlet and the biological carbon outlet are formed in the pyrolysis chamber; the grinding device is provided with a biological carbon inlet and a biological carbon powder outlet; the mixing tank is provided with a biological carbon powder inlet, an inlet of modified liquid containing phosphate and hydrochloric acid, a residual liquid outlet and a modified biological carbon outlet. The specific surface area of the modified biological carbon obtained by using the system can reach 480-550 m<2>/g, and when contaminated soil is restored with the modified biological carbon, the content of chromium in the soil can be reduced to 4.0 mg/kg or below.