The invention relates to a system and a method for producing chlorine-free phospho potash complex fertilizer using the method of low-temperature decomposition by fusion. The method comprises the main steps as follows: potassium chloride and concentrated sulfuric acid are mixed and reacted in a dechlorinating reaction tank, dechlorination is carried out at low temperature to obtain fused charge slurry; the melting charge slurry and powdered rock phosphate are mixed in a phosphorite decomposition tank for decomposition reaction to obtain slurry; and the obtained slurry is fed to a crasher afterbeing cooled to obtain chlorine-free phospho potash complex fertilizer. The powdered rock phosphate is decomposed completely under the state of anhydrous fusion without passivation; and the obtained phospho potash has slowly-releasing effect and is hard to be drained away after being applied in the soil, thereby supplying durable and balanced nutrition for crops. The invention has simpler and more direct technique process and low construction investment.