The cylindrical surfaces of potassium dihydrogen phosphate (KDP) single crystal grow very slow and the conical surfaces grow fast during the crystal growth under traditional method. A KDP single crystal with large cross section (8 mm×22 mm×45 mm) is gained at some constant temperature by vaporizing the solvent in the experiment. The experiment can be used for reference of KDP single crystal growth by pulling method. Compared with the traditional method, the growth rate along the cylindrical surface is improved effectively, especially the direction downward, which is on account of the dropping of liquid level and continued change of solute boundary layer. The experience in purifying and preheating the solution and choosing the initial temperature is accumulated. The experiment is also useful for future KDP crystal growth research.