NOVELTY - An active agent comprises ethanol extract of whole plant powder of Strychnos nux-vomica. The active component is prepared by warm soaking or cold soaking whole plant powder of Strychnos nux-vomica and ethanol organic solvent at volume ratio of 1:2-8 for 4-24 hours, filtering, and drying to obtain extract of whole plant powder of Strychnos nux-vomica; or repeatedly percolating and extracting the powder and ethanol at volume ratio of 1:2-8 for 2-10 times, or continuously refluxing or extracting for 4-24 hours, filtering, and drying.    USE - Active component for preparation of plant acaricide and miticide (claimed) for preventing or killing mites on cash crops including fruit trees and vegetables.    ADVANTAGE - The plant acaricide utilizing the active agent has high efficiency, low toxicity, and few residues, and is environment-friendly.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a method of preparing active agent of Strychnos nux-vomica, comprising dissolving extract (5-36 pts.wt.) of whole plant powder of Strychnos nux-vomica in solvent (30-95 pts.wt.); adding solubilizer (1-40 pts.wt.), emulsion and/or suspending agent (2-35 pts.wt.), synergistic agent (0.5-30 pts.wt.), or penetrant agent (0.5-28) to prepare plant acaricide. The solvent is water, chloroform, cyclohexene, ethanol, methanol, acetone, cyclohexanone, toluene, dimethylbenzene, sherwood oil, petroleum, or benzene. The solubilizer is sodium dodecylbenzene sulfonate, potassium dodecylbenzene sulfonate, Polysorbate(RTM: polyoxyethylene sorbitan fatty acid esters), polyglycol, polyoxyethylene sorbitol, lignosulfonate, tea seed cake, or Cassia nomame. The synergistic agent is tea saponin, dimethyl sulfoxide compound of formula 2-ClC6H4-N=OCH3SCH3, methylenedioxybenzene derivative, vinyldecyl aldehyde, or octachlorodipropyl ether. The emulsion is sodium acetate compound of formula C(15-17)H(31-37)COONa, sodium stearate, sodium dodecyl sulfate, N-dodecyl dimethylamine, polyoxyethylene ether, polyoxypropylene ether, polyoxyethylene paraoctyl phenol ether, fatty alcohol polyoxyethylene ether, polyoxyethylene fatty acid ester, or alkylaryl sulfonate. The penetrant agent is polyoxyethylene benzylphenol ether, azone, or triphenyl phosphine.