Dissolved carbon (DOC) in biochar is of great significance in soil improvement. DOC extracted from biochar is considered a suitable solution for the trend of automated irrigation in Vietnamese agriculture. Macadamia husk is the precursor used in the study. This study aimed to determine the optimal parameters for extracting DOC from macadamia husk-derived biochar and to investigate the properties of the extracted DOC in the solution. The investigated parameters include biochar type (based on pyrolysis temperature 300, 450, and 600℃), KOH content (0.5, 1.0, and 2.0 N), homogenization speed (4000, 6000, and 8000 rpm), extraction time (1, 2, and 3h), and biochar dosage (50, 100, and 150 g/L). The survey results determined the optimal extraction parameters, including biochar prepared at 300℃, KOH concentration of 1N, homogenization speed of 6000 rpm, extraction time of 2 hours, and biochar dosage: 100 g/L. The characteristics of DOC in the extract corresponding to the optimal extraction conditions were E2/E3 (0.6), indicating a high molecular weight; E4/E5 (2.8 < 3.5), suggesting that DOC was mainly composed of humic acid-like substances, and SUVA254 (0.78, the highest), indicating high stability. The research results provided a solution to use biochar to prepare DOC extract, a potential product made from agricultural by-products.