The invention discloses a water purification system and a water quality detection method and device thereof.The water quality detection method comprises the steps of detecting inlet water quality and outlet water quality of the water purification system so as to generate inlet water quality data and outlet water quality data respectively; acquiring K inlet water quality data and K outlet water quality data, calculating the difference value of every two adjacent inlet water quality data so as to generate (K-1) inlet water difference value data, and calculating the difference value of every two adjacent outlet water quality data so as to generate (K-1) outlet water difference value data, wherein K is an integer larger than or equal to 2; judging whether the water quality of the water purification system is abnormal according to any one inlet water quality datum, any one outlet water quality datum, any one inlet water difference value datum and any one inlet water difference value datum.In this way, whether water quality is abnormal is judged according to K inlet water quality data, namely pre-filtration water quality data, and K outlet water quality data, namely post-filtration water quality data, so that users can learn about water quality visually.