Yb3+/Er3+ codoped Ba5Gd8Zn4O21 up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol-gel method. UC spectra are composed of green emission from 2H11/2/4S3/2 → 4I15/2 transitions and red emission from 4F9/2 → 4I15/2 transition of Er3+ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level (4S3/2) lifetimes under excitation of 490 nm light, and the latter method was explained by the non-steady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er3+ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels (2H11/2/4S3/2) and red emitting stark levels (4F9/2(1)/ 4F9/2(2)) of Er3+ ion were estimated and the maximum sensitivity are 0.0032 K-1 at 490 and 0.0029 K-1 at 200 K in our experimental range, respectively. Moreover, the effects of UC emission color from different dopant concentrations and pulse widths of lasers on sensor sensitivity were also investigated in detail. Results imply that the present phosphor Ba5Gd8Zn4O21:Er3+/Yb3+ exhibits high and stable sensitivity in a wide temperature detection scope, which makes it an excellent candidate for an optical thermometer.