In order to explore the regulation effect of biochar on potassium leaching loss in fluvo-aquic soil and sandy soil, a soil column leaching simulation test was conducted, and water washed biochar was used as the research object, the contents of different forms of potassium, surface morphology and oxygen-containing functional groups in biochar before and after washing were compared, and then explore the regulation effects of different amounts of washed biochar on potassium leaching loss in two types of soil. The results showed that the water-soluble potassium content in biochar was decreased from 13.9 g/kg to 0.06 g/kg by water-washed treatment, but the change of pore structure and surface oxygen-containing functional groups of biochar were less affected. Moreover, the effect of water-washed biochar on water leaching loss of fluvo-aquic soil and sandy soil was influenced by its application amount, which showed a trend of low amount of water-washed biochar promoted the water leaching loss, but high amount inhibited the water leaching loss, and adding 1% water-washed biochar significantly increased the total amount of water leaching loss in the two types of soil. The effects of biochar on potassium leaching in two types of soils were different. Adding 2% and 4% washed biochar showed significant inhibition and control effect on potassium leaching in sandy soil, and compared with the treatment without biochar, the total leaching amount of potassium decreased by 21.2% and 28.3%, respectively. While adding 1% washed biochar increased the total amount of potassium leaching in fluvo-aquic soil. In addition, biochar could improve soil cation exchange capacity and exchangeable potassium content of topsoil, and the increasing extent improved with the increasing of biochar application amount, and the improvement effect was more obvious in sand soil. Therefore, for the perspective of potassium leaching, biochar was more suitable for sandy soil with low cation exchange capacity and poor fertilizer retention capacity.