Carbon nanotubes (CNTs) were synthesized on a Ni catalyst-diatomite substrate using a chemical vapor deposition (CVD) method. By controlling the synthesis conditions, we explored the influence of synthesis time and temperature on the synthesis of CNTs. The carbon yield and microstructure of the resulting CNTs were investigated by thermogravity (TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy analysis. Then, two kinds of purification methods, annealing and acid treatment, were used on the synthesis products, diatomite-CNT, and can effectively increase the specific surface area of the product. The adsorption behaviors of diatomite-CNT composite adsorbents for phenolic compounds have been investigated. The adsorption experiments showed that the adsorption process reached equilibrium quickly, while the adsorption kinetics followed the pseudo-second-order model and the adsorption isotherm followed the Langmuir model well, respectively. The thermodynamics parameters indicated that the adsorption behavior is an exothermic process performing spontaneously. The diatomite-CNT adsorbent will perform well for removal of phenolic compounds from wastewater.