Abstract: The luminescence accompanied in the visible region was found by experimental study of Yb3+ doped fiber laser. The spectrum of the green luminescence was measured with a spectrometer, which shows that the wavelengths of luminescence are 639.4nm, 520.7nm, 487nm and 474.8nm. Analysis shows that the concomitant red, green and blue luminescence is the result of the cooperation of Yb3+ and Pr3+. The fluorescence process is to transform the energy from Yb3+ ions to Pr3+, and exited Pr3+ induces red, green and blue luminescence. It is concluded from the experiment result that the luminescence produced after Yb3+ doped fiber laser absorbs pumped light is upconversion, which may degrade the transform efficiency of the laser energy of fiber lasers.