Monitoring on the melt pool light intensity is one important method to process monitoring in selective laser melting(SLM). A melt pool light intensity monitoring system, consisting of hardware and software system, was established for the SLM forming process. By near infrared filter technology and photodiode detection circuit, the data of melt pool light intensity in the forming process were obtained. As the change of the melt pool light intensity in the different forming positions of laser act can′t be visualized by the time domain signal, the mapping algorithm was proposed to model the data of the melt pool. Data analysis and modeling of the melt pool light intensity under the variable power were conducted, when the laser power was 120 W and the melt pool temperature was 1 404 ℃, the mean value of the diode signal was 0.31 V, the standard deviation was 0.04 V, the melt pool was stable; when the power increased to 220 W and the melt pool temperature was 1 727 ℃, the diode signal mean increased to 0.81 V, the standard deviation increased by 0.22 V, the stability of the melt pool deteriorated. The results show that the influence of different process parameters on the thermal radiation behavior of melt pool can be obtained through the melt pool light intensity monitoring system, which can be used to support the development and optimization of SLM forming process.