The radiation-mode light radiated from an M-Z optical waveguide may couple with the fiber, and this is the main factor leading to failure of the high-precision coupling between a chip and the fiber array. This study used fiber to scan the waveguide and radiation-mode light with a high-precision coupling method and studied the center position of the radiation-mode light regions, the characteristics of the radiation-mode light power that changed with the bias voltage, and the power distribution characteristics of the waveguide and radiation-mode light scanned by the fiber. The effective coupling ranges of the waveguide in the X and Y directions were 14~15.5 and 14~16μm, respectively, and the effective coupling ranges of the radiation-mode light in the X and Y directions were 89~92 and 92~96μm, respectively. The study investigated the characteristics of the radiation-mode light to formulate the correct solution and solve the coupling failure caused by radiation-mode light.