Technique of residual cladding pump light detection in active fiber was researched, which was used in assembling the fiber laser system. With this technology, a fiber probe was used to detect the pump light leaking from the cladding with fiber′s coating on and stripping. It′s found that there was a linear relationship between leaking light power and pump light power, and the proportionality factor was measured. Therefore, it does not need to cut and fuse the active fiber for many times to get the best length of active fiber, and it can save the measuring period and experiment expense a lot. Two types of fiber probes were used, one was taper fiber probe and the other was ramp plane fiber probe. The testing results of low light using these two fiber probes were given, and it′s found that taper fiber probe is better than ramp plane fiber probe. In order to check the accuracy of fiber probe, a laser test has been done. By testing the power change before and after cutting off optic fiber, and by testing the power via fiber probe at the same position as the cut off point, it proves that fiber probe can well predict the residual pump light.