The upper limit of output power for single-mode laser from a single ytterbium-doped fiber can be reached using a tandem pumping configuration. Ytterbium-doped fiber laser is difficult to operate below 1030 nm due to the relatively large absorption cross sections. In order to get highly efficient ytterbium-doped fiber laser, the most basic method is to regulate the fluorescence properties of fiber and make the emission subpeak approach 1018 nm. The mechanism of how Yb3+ ions fluorescence properties change in silica fiber is studied and a 1018 nm ytterbium-doped fiber is obtained by co-doped method. The emission subpeaks of the preform and fiber are at 1008 nm and 1021 nm, respectively. The emission subpeak of the fiber is 18 nm, which is shorter than that of normal Yb-doped fiber, and the difference of emission cross sections is larger. Its optical-optical convertion efficiency at 1018 nm is up to 68%.