The pulse current method is a new method which can measure the junction temperature and thermal capacity of LED simultaneously. By injecting constant short pulse of square-wave current to the LED, the pulse amplitude is equal to rated current, and the LED’s forward voltage is measured at different junction temperatures. Then, the sensitivity coefficient of forward voltage and junction temperature is obtained under a rated current. Therefore, by measuring the forward junction voltage under the rated current, users can obtain the junction temperature. Finally, by using the junction temperature variation and single pulse of heat input, the thermal capacity can be calculated. This paper studied the influence of pulse width on the measurement of junction temperature and LED’s thermal capacity, and pulse current measurement devices are designed. The results show that, in order to achieve the effect of pulse current on junction temperature measuring accuracy lower than 1℃, the pulse width should be controlled from a few to a dozen μs, and the thermal capacity measurement of using pulse current or thermal integration period is basically consistent.