An optical device is proposed to collect the thermal radiation generated by laser cutting in this paper. The device can be added into the optical path of laser cutting without affecting the normal transmission of the laser. At the same time, it can monitor the laser penetration and cutting process. First of all, ZEMAX is used to design the optical path. Through a special mirror, the thermal radiation is reflected from the laser light path to the outside, and finally enters a sensor well. Then, the optical device of collecting thermal radiation is added into the laser cutting head by using a 3D software. Finally, the results are show by using the TRACEPRO. The software can be used to observe the thermal radiation light collected by the sensor when the cutting head is working. From the actual test curve of the output, the optical device can clearly judge the laser penetration time point and the abnormal situation during cutting, which shows that the design method is feasible. The device can be used to improve the cutting quality and protect the expensive laser.