The temperature of the molten pool is one of the most important parameters in the process of metal forming based on selective laser melting. To ensure the closed-loop control of molten pool temperature, this study implementes an online detection of molten pool temperature based on the colorimetric temperature measurement method and photoelectricity technology. It proposed a type of composite operational amplifier that successfully detected the molten pool radiation signal with small amplitude and fast change. An embedded system was built to upload the radiation data of the molten pool to the industrial computer of metal forming equipment. A method of colorimetric temperature measurement was employed to eliminate the effects of the laser incident angle and other agents on the detection of radiation intensity, and furthermore, to obtain the temperature of the molten pool. During the process of melting 90%Cu-10%Sn alloy powder, it is found that the band with the strongest radiation in the molten pool of the material is 580--590 nm within the spectral range of 540--660 nm. Under the laser power range of 40--200 W, the estimated molten pool temperature range is 700--1700 ℃. The online detection of molten pool temperature during metal forming based on selective laser melting provides feedback information of molten pool temperature for the control system of metal forming equipment. Furthermore, the proposed approach of online detection also can be considered as a reference to upgrade the equipment.