The invention discloses a device and method for measuring the junction temperature of an LED through a bridge. The device comprises a power supply E and the Wheatstone bridge which is composed of a variable resistor R1, a variable resistor R2, a variable resistor R3 and the LED. The device further comprises a fast change-over switch S1 and a fast change-over switch S1', whether the bridge is turned on or off is controlled through the switch S1, the LED is connected with a working power supply through the switch S1', and a direct current voltage amplifier is further connected to the middle of the bridge. The method comprises the steps of calibrating K and measuring the junction temperature of the LED under a working current. It is not requested to accurately measure voltage at the two ends of the LED within 10<-5> second, whether the reading on a voltage comparator is zero needs to be observed only, the degree of precision depends on the flexibility of the voltage comparator, and influences of voltage pulses and other errors can be avoided as long as the voltage comparator is selected reasonably. The device and the method can be used for measuring the junction temperatures of LEDs under various working currents such as alternating currents and pulses and are high in adaptability. According to the device and the method, errors of junction temperature measurement can be effectively controlled by prolonging testing time.