A current sensing circuit used in a buck-boost converter having a pair of buck switches and a pair of boost switches, including: a first sensing circuit providing a detection current though a first normally-ON transistor and a second normally-ON transistor, and a second sensing circuit detecting an average of the detection current and providing a current sensing signal in accordance with the average. During a turn ON time of a first low side switch of the pair of buck switches, the detection current represents a current flowing through the first low side switch, the current sensing signal represents an output current. During a turn ON time of the second low side switch of the pair of boost switches, the detection current represents a current flowing through the second low side switch, and the current sensing signal represents an input current.