A temperature sensing device includes a bandgap voltage generator, N mirror current sources, a temperature voltage generator, and a temperature calculating unit. The mirror current sources mirror N mirror currents according to a positive temperature coefficient current. The temperature voltage generator sets the conducting number M of the mirror current sources based on a control signal, so as to generate a temperature voltage. The temperature calculating unit gradually counts the control signal and compares a potential of the temperature voltage with a potential of a reference voltage generated by the bandgap voltage generator after counting the control signal, so as to calculate and obtain temperature information. Thus, the temperature sensing device controls the conducting number M of the mirror current sources to generate the temperature voltage instead of applying serially-connected resistors, so as to reduce a circuit area of the temperature sensing device and reduce noise.