Thermal performance is one of the main factors which affect the optical and electrical performance of high power LED. The thermal performance testing system for high power LEDs based on voltage-current characteristics with pulse injection was designed in this paper, which can test the relationship between the operating current and forward voltage of LED under different junction temperatures, thereby obtaining LED thermal characteristics parameters. The system worked by generating a controlled narrow pulse current to drive the LED, and sampling the peaks of voltage and current of LEDs with LED heat sink temperature control and acquisition, thereby obtaining the voltage-current characteristic curve in different junction temperatures. Compared with other voltage-current testing systems, the designed system adapts the narrow pulse duty cycle(1 μs), so the PN junction of LED devices is always in the process of alternately heating and cooling, which can avoid large heat accumulation and greatly improve the accuracy of measurement. In the experiment, a power LED device was tested by the system and the voltage-current-temperature curve was obtained. Then the B-spline based U-I-T model of the LED was established, so the real-time online detection of LED device was achieved.