Abstract: Humic acid fertilizer has been one of the most important multifunctional fertilizers, particularly for the recyclable organic resource. There is an outstanding emulsifying and dispersing effect of humic acid fertilizer as protein feed on pesticides for the wide application prospect. Drying has widely been a necessary step for the processing of humic acid fertilizer for the high quality products. However, the slow heat transfer, low heating uniformity, and low efficiency often occur in the commonly-used drying for the humic acid fertilizer. Fortunately, radio frequency (RF) vacuum drying can be widely expected to combine RF heating and vacuum. An RF vacuum drying can be used to keep the material in a high frequency alternating electromagnetic and vacuum field, where the electromagnetic wave between electrodes can produce the heat inside the material through the friction caused by the charged ion shift and dipole rotation, leading to the significantly improved heating speed, heating uniformity, moisture migration rate, and penetration depth for the high drying efficiency. The vacuum environment can also be beneficial to reducing the color changes and the loss of important components during drying. This study aims to investigate the RF vacuum drying characteristics and the quality of humic acid fertilizer. The heating rate and heating uniformity of humic acid fertilizer were recorded at different electrode gaps (160, and 165 mm) and initial moisture content (10%, 15%, and 20% w.b.). An RF vacuum drying experiment was carried out, where the hot air drying was treated as the controlled group. A Weibull model was selected to fit the drying characteristic curves of a humic acid fertilizer. A comparison was made on the performances of RF vacuum and hot-air drying on the color, total nitrogen, total phosphorus, total potassium, and organic matter in the humic acid fertilizer. The results indicated that the better heating uniformity, the higher heating, and drying rate were achieved under 0.085 MPa vacuum degree, 160 mm electrode gap, and 15% initial moisture content. Temperature-time curves of humic acid fertilizer presented the inflection point at 53 ℃, where the heating rate before 53 ℃ was higher than after 53 ℃. Except for the material with 20% moisture content, there was no significant effect of the electrode gap on the heating uniformity in the top layer of humic acid fertilizer (P>0.05), but a significant influence of initial moisture content was found (P<0.05). The time required for the RF vacuum drying was about 178 min shorter than that of hot air drying, indicating that the RF vacuum drying could significantly improve the drying rate (P<0.05). The Weibull model was also used to accurately simulate the RF vacuum drying process. The scale parameters and shape parameters decreased with the decrease of electrode gap, but first decreased and then increased with the increase of initial moisture content. Effective diffusion coefficients Deff increased with the decrease of electrode gap, and first increased and then decreased with the increase of initial moisture content. Meanwhile, in general, neither electrode gap nor initial moisture content had a significant influence on the color, total nitrogen, total phosphorus, total potassium, and organic matter of humic acid fertilizer (P>0.05). By contrast, the humic acid fertilizer that was dehydrated by hot air drying at 65 ℃ presented a significantly higher ΔE value, but a lower total nitrogen and organic matter content, compared with RF vacuum drying (P<0.05). An optimal combination of RF vacuum drying parameters was achieved, where a 160 mm electrode gap, and 15% initial wet basis moisture content. This finding can provide the theoretical basis for the application of RF vacuum drying of a humic acid fertilizer.