The effect of crystallinity degree of MoSe₂ on the potassium ions storage performance in potassium-ion batteries (PIBs) has been largely overlooked in the energy communities. In this study, we experimentally realize MoSe₂ grown on graphene nanoribbons (MoSe₂-GNR) with tunable crystallinity by tailoring the thermal annealing temperature, and further investigate the effect of crystallinity degree in MoSe₂-GNR on the potassium ions storage performance. The spectral, electrochemical, and microscopy experiments indicate that high-temperature thermal annealing results in a high crystallinity degree of MoSe₂-GNR with decreased interlayer spacing of (002). The MoSe₂-GNR with high crystallinity degree exhibits a high capacity, but suffers from reduced cycling stability. What is more, the in-situ X-ray powder diffractometer (in-situ XRD) and in-situ Raman experiments reveal the phase transition in MoSe₂ triggered by potassium ions insertion/extraction during the potassium ions storage. The work sheds light on the development of MoSe₂-based anode materials for PIBs.