The cladding light in double clad-fiber laser will not only affect the laser beam quality, but also damage the optical devices. The reasons of the generation of cladding light in the passive double-clad fiber are analyzed, and the influence of these reasons on the cladding light generation is studied experimentally by the method of power and optical field distribution detection. The results indicate that, low-quality splices and mode field mismatch between the passive double-clad fibers would raise the coupling between fundamental mode and cladding modes, while the fiber combiner and isolator would lead to the leakage of light from the core to the cladding, which results in the generation of cladding light. Notes of restraining the generation and reducing the influence of cladding light are discussed.