Radiation temperature could be quantified in indirect-drive ICF experiments. This paper introduces a new method for calculating radiation temperature without unfolding. It is a simple method which can provide the real time temperature. Radiation temperature obtained by this method is in accordance with that calculated by traditional methods with an uncertainty of about 7%. The result by measuring shock wave’s velocity has proved this method to be correct. With this method, we have built a new data processing system for 15-channel soft X-ray spectrometer.