The invention discloses a boiler system with a U-shaped flue and a boiler water charging system. The boiler system comprises a boiler body, a boiler flue and a U-shaped flue device, wherein the U-shaped flue device is connected to the boiler body through the boiler flue, the U-shaped flue device comprises a front end vertical flue, a heat exchanger and a rear end J-shaped flue, the front end vertical flue is connected to the boiler flue in a sealing mode, the fume which is discharged by a boiler flows from top to bottom in the front end vertical flue, the heat exchanger is connected to the front end vertical flue in a sealing mode, the fume which is discharged by the front end vertical flue flows the water pipe of the heat exchanger from top to bottom, the rear end J-shaped flue is connected to the heat exchanger in a sealing mode, the fume discharged by the heat exchanger flows from bottom to top in the rear end J-shaped flue and is discharged, and the rear end J-shaped flue, the front end vertical flue and the heat exchanger are connected to form a U shape. The problems that the heat exchanging is not complete due to quick flue speed, the flue is corroded, and the condensate is recovered are solved. The residual heat of the fume can be recovered deeply.