NOVELTY - Extraction and purification of catalpol comprises pretreatment of fresh Rehmannia root tuber; adding calcium carbonate or barium hydroxide in the plant material to restrain the activity of enzyme and neutralize the vegetable acid; extracting with ethanol; separating by active carbon column chromatography; separating by silica column chromatography; crystallizing using acetone-water as crystallizing solvent; and performing solid-liquid separation and washing operations.    USE - Extraction and purification of catalpol (claimed).    ADVANTAGE - The method is simple, convenient, environmentally friendly, energy-saving, and suitable for industrial production. It provides catalpol product with high purity.    DETAILED DESCRIPTION - Extraction and purification of catalpol by using active carbon column chromatography and crystallization comprises: (A) pretreatment of medicinal material comprising taking fresh Rehmannia root tuber; cleaning and removing the non-medicinal part, impurity, and parts with mildew and dust bits; washing or rinsing to remove the silt; and slicing into sheets or small pieces; (B) solvent extracting comprising adding calcium carbonate or barium hydroxide in the plant material to restrain the activity of enzyme and neutralize the vegetable acid; adding ethanol, which is 4-5 times of the raw material amount; extracting at 3000-8000 revolutions/minute (rpm); using laundry drier to centrifuge and primarily separate the plant broken tissue; filtering and extracting liquid; concentrating; and drying into dry extract; (C) separating by active carbon column chromatography comprising adding water to the product obtained in the extraction step; dissolving and filtering; passing through well-processed separating column, which is processed by heating granular active carbon for 4.5 hours at 450 degrees C, adding distilled water to soak for 1 hour, removing the suspending powder, and using distilled water to pack column by wet method; washing with water until the effluent liquid is nearly colorless and sugar reaction test is negative (negative Molisch reaction); combining with thin layer chromatography (TLC) inspection until catalpol occurs; using 75% ethanol to wash until no more catalpol is detected; combining the alcohol eluent; recycling the ethanol and concentrating; and drying to obtain dry extract; (D) separating by silica column chromatography comprising adding methanol or ethanol to dissolve the dry extract; filtering; adding column chromatography silica gel and sampling; drying; putting the sample at the top end of chromatography column; eluting by ethyl acetate-ethanol (8:2), performing TLC detection and collecting; combining the eluate including catalpol; recycling the solvent; and drying the residue at 60 degrees C; and (E) crystallizing comprising taking the acetone-water (1:1) as crystallizing solvent; using methods such as primary crystallization (including cooling crystallization, evaporation crystallization, vacuum crystallization, and reaction and salting-out crystallization); recrystallizing; fractional crystallizing for batchwise (intermittent) operation or continuous operation; and performing solid-liquid separation and washing operations to obtain catalpol product with high purity.