The invention discloses a method for preventing and/or improving water blackening and smell and a cleaning agent adopted by the method. The method comprises a step of adding a water cleaning agent at a water inlet; and the water cleaning agent contains the following components in percentage by mass: 20-60% of modified negative ion powder, 10-20% of silica sol and 20-70% of modified medical stone, wherein the modified negative ion powder is prepared by the following steps: calcining negative ion powder, adding dodecyldimethylbenzyl ammonium chloride and acetic acid and stirring for reacting, and finally grinding to obtain the modified negative ion powder; and the modified medical stone is prepared by the following steps: calcining medical stone, adding citric acid and stirring for reacting, and finally grinding to obtain the modified medical stone. Based on the principle of intercepting pollution from the source, the invention provides a method for preventing water blackening and smell from the source and a cleaning agent for the first time, and water blackening and smell are effectively prevented or delayed.