NOVELTY - Preparation of biodiesel using oil with high acid value comprises adding oil with high acid value into solid catalyst, dripping crude glycerol into oil with high acid value, and esterifying; treating the esterified oil by alcoholysis; treating biodiesel by absorbing, decolorizing, and refining; and recycling crude glycerol by adding sulfuric acid into glycerol layer, heating to neutralize, recycling methanol, standing, and centrifuging.    USE - Preparation of biodiesel (claimed).    ADVANTAGE - The method uses crude glycerol instead of refined glycerol, where cost of crude glycerol is 50% of the cost of refined glycerol. The crude glycerol obtained by alcoholysis can be used as reactant for esterifying, so that the cost of the raw material is saved. The usage amount of catalyst for esterification is low, i.e. 20-40% of the amount of solid catalyst of the prior art. The catalyst used by the method has high catalytic efficiency. By absorbing, decolorizing, and removing soap, the step of washing with water is removed, so that there is no wastewater produced. The method meets the green processing requirement. It is environment friendly.    DETAILED DESCRIPTION - Preparation of biodiesel using oil with high acid value comprises:    (A) taking oil with high acid value as raw material; adding into solid catalyst; dripping crude glycerol into oil with high acid value; under vacuum condition, refluxing to esterify; controlling the temperature of the cooling water in a reflux condenser; taking out the water produced in esterification; condensing and refluxing glycerol; centrifuging or standing the esterified product; and separating into layers to obtain esterified oil on the upper layer, unreacted crude glycerol on the middle layer, and solid catalyst and impure salt on the lower layer;    (B) treating the esterified oil by alcoholysis comprising mixing esterified oil with methanol; adding into alkaline catalyst; carrying out alcoholysis; after finishing reacting, standing; and centrifuging to separate into layers and obtain biodiesel on the upper layer and glycerol on the lower layer;    (C) treating the biodiesel by absorbing, decolorizing, and refining, comprising heating the biodiesel layer to recycle methanol; adding into absorbing agent; decolorizing; and removing the soap to obtain refined biodiesel; and    (D) recycling crude glycerol by adding sulfuric acid into glycerol layer; heating to neutralize; recycling methanol; after finishing reaction, standing; and centrifuging to separate into layers and obtain crude biodiesel on the upper layer, crude glycerol layer on the middle layer, and sulfate on the lower layer.