A back carrier adjusting system is provided in the present disclosure. The back carrier adjusting system is applied to a back carrier, which includes a main body and a strap unit connected to the main body. The back carrier adjusting system includes a guiding unit, a sliding unit and a fastening device. The guiding unit is disposed on the strap unit. The sliding unit is connected to the main body and slidably limited in the guiding unit. The fastening device includes at least one lace and a reel mechanism operably coupled with the at least one lace. The at least one lace is connected to the sliding unit. The reel mechanism is operated to retract or release the at least one lace, which makes the sliding unit move relative to the guiding unit, in order to adjust a height of the main body relative to the strap unit.
