The invention discloses a direct rectification method of molten liquid crude cadmium. A cadmium cake is placed in a crude cadmium smelting furnace to be smelted, the smelting temperature ranges from 500 DEG C to 700 DEG C, the adding proportion of sodium hydroxide is (15-16):100, the adding proportion of a reducing agent is (1-2):100, and impurities Zn, Pb, Cu and NaOH react to generate sodium salt which is generally called alkali slag; the density of the alkali slag is smaller than that of cadmium, so that the alkali slag is layered and floats on cadmium liquid; meanwhile, a small amount of CdO wrapped by the alkali slag is reduced into metal cadmium by a reducing agent, the alkali slag is cleared up regularly to remove impurities, and a crude cadmium solution containing larger than 98% of Cd and smaller than 0.05% of Zn is obtained; the crude cadmium solution passes through the bottom of a crude cadmium smelting furnace, flows out of a discharging opening controlled by a valve special for groove-free metal melt and flows into a middle maintaining furnace; the crude cadmium solution in the middle maintaining furnace is sucked into a vacuum rectifying furnace through a siphon tube of the vacuum rectifying furnace for high-temperature rectifying at the temperature of 400 DEG C to 500 DEG C; due to the fact that the volatilization temperature of cadmium is far lower than that of impurity elements, cadmium vapor is formed by liquid cadmium, and is condensed into liquid cadmium or a fine cadmium ingot in a condenser.