The invention relates to a method and an apparatus for preparing trichlorosilane through reaction rectification by using proportionate reaction. The method comprises the following steps of: reacting dichlorosilane with silicon tetrachloride in the upper middle part of a reaction rectification tower, obtaining incompletely reacted dichlorosilane at the top of the reaction rectification tower; reflowing at the top of the tower and in the middle of the tower, obtaining a mixture of dichlorosilane, trichlorosilane and silicon tetrachloride at the bottom of the tower, entering a light-end removal tower to remove a light component, obtaining the dichlorosilane at the top of the light-end removal tower; reflowing to the reaction rectification tower for recycling, obtaining a mixture of trichlorosilane and silicon tetrachloride at the bottom of the tower, entering a heavy-end removal tower to separate the trichlorosilane from the silicon tetrachloride, wherein the trichlorosilane at the top of the heavy-end removal tower is a product; and taking out the silicon tetrachloride at the side line of the lower part of the tower, entering the reaction rectification tower for recycling, and obtaining excessive silicon tetrachloride at the bottom of the tower. In the invention, by utilizing the proportionate reaction, the utilization ratio of the raw materials is enhanced; the problem of dichlorosilane and silicon tetrachloride enrichment is solved; and the final conversion ratio of the dichlorosilane is ensured to reach 100 percent theoretically.