In this work, Gd _20+2xHo _20–xEr _20–xCo ₂₀Ni ₁₀Al ₁₀( x= 0, 5, 10) high-entropy metallic glasses (MGs) with a critical diameter of 2 mm are successfully designed and fabricated by substituting Gd, Ho and Er. The effects of type and content of rare-earth (RE) elements on the microstructure, thermodynamic behaviors, and magnetocaloric effect (MCE) are investigated systematically. The amorphous structures of the ribbons and as-cast rods are confirmed by X-ray diffraction (XRD) with Cu Kα radiation (2 θ= 20°–80°). The atomic-scale ordered configurations are examined by using high-resolution transmission electron microscope (HRTEM). Thermal analysis is carried out on differential scanning calorimeter (DSC) with a heating rate of 20 K/min by using ribbons. The magnetic measurements are conducted by using magnetometer in a temperature range of 5–180 K. According to DSC traces, it is suggested that as Ho and Er are replaced by Gd, the thermal stability of MGs slightly decreases, for example, both glass transition temperature ( T _g) and initial crystallization temperature ( T _x) decrease gradually, meanwhile the liquidus temperature ( T _l) increases, which results in a reduction of glass-forming ability criteria such as the reduced glass transition temperatures T _rg( T _rg= T _g/ T _l), γ( γ= T _x/( T _g+ T _l)), and γ _m( γ _m= (2 T _x– T _g)/ T _l) ,thermodynamically. The analyses based on XRD and HRTEM show that the degree of order in MGs decreases with Gd content increasing, which facilitates the glass formation. The magnetocaloric parameters such as Curie temperature ( T _c), maximum magnetic entropy change ($ | {\Delta S_{\text{M}}^{{\text{pk}}}} | $) and relative cooling power (RCP) all increase gradually with the addition of Gd. The Gd ₄₀Ho ₁₀Er ₁₀CoNiAl exhibits the best refrigeration performance in all studied systems, where the peak value of $ |{\Delta S}_{{\mathrm{M}}}| $ is 8.31 J/(kg·K) and RCP is 740.82 J/kg. The results indicate that MCEs of MGs including RCP, T _cand $ | {\Delta S_{\text{M}}^{{\text{pk}}}} | $, mainly depend on the de Gennes factor rather than the effective magnetic moment, while thermodynamic properties are more affected by the f-d hybridization effect. As the number of 4f electrons increases, the thermal stability increases with the degree f-d orbital hybridization increasing. In summary, the RE-based MG with high thermal stability and adjustable T _ccan be achieved by the RE substitution via adjusting the number of 4f electrons.