The optimal preparation conditions for cellulose/diatomite composite adsorbents (CDAs) were determined to be a diatomite concentration of 40 wt%, a holding temperature of 700 °C, and a holding time of 30 min. Meanwhile, the yield of CDAs was 70.6%, and the methylene blue (MB) adsorption value was 191.8 mg/g. The specific surface area, total pore volume, and average pore size of the CDAs were 489 m2/g, 0.372 cm3/g, and 3.04 nm, respectively. It was inferred that the diatomite was cross-linked with the cellulose carbon to form some micropores. Greater MB initial concentrations and pH values improved the adsorption on the CDAs. The MB adsorption behavior of the CDAs was described very well by the pseudo-second-order model. The isotherm models showed relatively high adsorption of MB.