The invention relates to a continuous production method of powdery and granular water-soluble ammonium polyphosphate. The method comprises steps as follows: 1) prepolymerization: a phosphorus source and urea are added to a prepolymerization device respectively and react at the temperature of 60-140 DEG C for 2-10 min, and a liquid material is obtained; 2) thermal polymerization and cooling: the liquid material is added to a belt type polymerizer provided with a heating zone and a cooling zone at a constant speed, and a solid material is obtained through heating, polymerization, cooling and curing sequentially; 3) crushing: powdery and granular water-soluble ammonium polyphosphate is obtained after the solid material is crushed. Compared with the prior art, the prepared viscous liquid material with proper flowability is prepared firstly, then, the liquid material sequentially passes by the heating zone and the cooling zone with movement of a driving belt, the solid material is obtained, continuous production of ammonium polyphosphate is realized, the problems such as the difficulty in continuous discharge, difficulty in crushing and the like in a conventional ammonium polyphosphate production process are solved, the production efficiency is high, and the product performance is easy to control.