According to the characteristics of most high power laser diode array/stacks only working at quasi continuous wave(QCW), the calculation method of temperature rising by measuring the spectra for thermal resistance calculation is recommended, which is simple, direct, and fast. Requirements of continuous wave operation which are conventionally needed at calculating the thermal resistance are avoided. The thermal resistance, which is calculated at different duty cycles and the same working current is, well consistent and closer to the values given in previous references. The calculation methods have actual application values in manufacture process.