It is well recognized that a widely wavelength-tunable mid-infrared (MIR) fiber laser plays an important role in the development of compact and efficient coherent sources in the MIR range. Herein, the optimizing Er/Ho ratio for enhancement of broadband tunable MIR emission covering 2.6-2.95m in the Er3+/Ho3+-codoped transparent borosilicate glass-ceramic (GC) fibers containing NaYF4 nanocrystals under 980nm excitation was investigated. Specifically, the obtained GC fibers with controllable crystallization and well fsd-maintained structures were prepared by the novel melt-in-tube approach. Owing to the effective energy transfer between Er3+ and Ho3+ after crystallization, the 2.7m MIR emission was obviously enhanced and the emission region showed a notable extension from 2.6-2.82m to 2.6-2.95m after the addition of Ho3+. Importantly, we conducted a theoretical simulation and calculation related to the MIR laser performance, signifying that the GC fiber may be a promising candidate for MIR fiber laser. Furthermore, the melt-in-tube approach will provide a versatile strategy for the preparation of diverse optical functional GC fibers.