Low dissolved oxygen biofilm reactor was used to treat the mixed wastewater in a village in north bank of Lake Dianchi. The purpose of the operation was to remove the solid waste, which was difficult to settle down naturally, so forth tofacilitatethe next step of wetland purification. A field experiment was undertaken for 12 months in nature conditions. The result showed that when the hydraulic loading rates between 3 and 12m³/(m²·d), Air/Water ratio between 1:1 and 2:1, DO was controlled between 0.1 and 2.0mg/L, the reactor could remove the organic matter efficiently. When the loading rates of COD under 0.58kg/(m³·d), removal rates were above 60%. Low dissolved oxygen slowed the desquamation of biofilm and its thickness was reached to 800μm. Process of obstruction and adsorption was enhanced for SS removal and the average removal rate was 90.7%. The experiment found that there was still some removal effect of nitrogen and phosphorous in low dissolved oxygen in the reactor. Energy cost was significantly reduced by decreasing aeration. At the flux of 4000t/d, the calculated treatment expense was 0.05-0.1RMB/t. Although the concentration of COD and SS of mixed wastewater was greatly fluctuated, the outflow quality and biofilm growth was relatively stable. It's suitable for further stable disposal and the treatment of suburb low concentration mixed wastewater. It provided a new technical for wastewater control in suburb area of Dianchi basin.