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 were successfully designed and fabricated by the substitution of Gd, Ho and Er. The effects of types and contents of rare earth (RE) elements on the microstructure, thermodynamic behaviors, and magnetocaloric effect (MCE) were investigated systematically. The amorphous structure of the ribbons and as-cast rods were confirmed by X-ray diffraction (XRD) with Cu Kα radiation (2 θ= 20°-80°). The atomic-scale ordered configurations were examined by using high-resolution transmission electron microscop (HRTEM). Thermal analysis was carried out on differential scanning calorimeter (DSC) with a heating rate of 20 K/min by using ribbons. The magnetic measurements were conducted by using magnetometer in the temperature range of 5-180 K. According to DSC traces, it suggests that as Ho and Er are replaced by Gd, the thermal stability of MGs slightly decreases, e.g., 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 ₁)、 γ( γ= T_x/( T _g+ T ₁))和 γ _m( γ _m= (2 T_x- T _g)/ T ₁), thermodynamically. The analyses based on XRD and HRTEM show that the degree of order in MGs decreases with increasing Gd content, which facilitates the glass formation. The magnetocaloric parameters such as Curie temperature ( T _c), maximum magnetic entropy change (|ΔS _M ^pk|) and relative cooling power (RCP) all increase gradually with the addition of Gd. Gd40Ho10Er10CoNiAl exhibits the best refrigeration performance among all studied systems, where the peak value of |Δ S_M| is 8.31 J·kg ^-1·K ^-1and RCP is 740.82 J·kg ^-1. The results indicate that MCEs of MGs including RCP, T _cand |ΔS _M ^pk|, mainly depend on the de Gennes factor rather than the effective magnetic moment, while thermodynamic properties are more affected by the f-dhybridization effect. With the increase of 4 felectrons, the thermal stability increases with increasing the degree f-dorbital hybridization. In summary, the RE-based MG with high thermal stability and adjustable T _ccan be achieved by means of the RE substitution via adjusting the number of 4 felectrons.