The present invention provides a transistor circuit with protecting function. The transistor circuit includes a transistor, a voltage detecting unit, a current detecting unit, a temperature detecting unit, and a protecting unit. The voltage detecting unit detects a voltage drop of the transistor and generates an over-voltage protection signal. The current detecting signal detects a current flowing through the transistor and generates an over-current protection signal. The temperature detecting unit detects a temperature of the transistor circuit and generates an over-temperature protection signal. The protecting unit is coupled to the control terminal to control a state of the transistor according to the over-voltage protection signal, the over-current protection signal, and the over-temperature protection signal to reduce the voltage difference between the control terminal and the second terminal, such that the voltage drop of the transistor is decreased or decreased to zero.