Biochar (BC) returned to soil will release amounts of dissolved organic matters (DOM), which can change soil-DOM's content and chemical properties and then have important impacts on their environmental behaviors. The molecular composition and structures of BC DOM would determine their complex environmental behaviors. However, the studies on chemical composition, especial on molecular weight (MW) separated fractions, are still limited. In this study, two types of biochar were firstly produced by pyrolyzing rice straw, and pig manure at different temperatures (300, 400 and 500 ℃), and then DOM therein was fractionated into three MW fractions, including <1, 1~5 and >5 kDa, using ultrafiltration method. Subsequently, the content and optical properties of MW fractions were investigated based on dissolved organic carbon (DOC), ultraviolet-visible (UV-vis) spectra, and excitation-emission matrix spectra combined with the regional integral protocol (EEM-FRI). The results showed that the proportional DOC distributions of <1, 1~5 and >5 kDa into bulk BC DOM were 42%~60%, 16%~23% and 23%~29%, respectively. The corresponding distributions on α₂₅₄ were 4%~27%, 8%~49% and 26%~81%. These results suggested that the major OC species in bulk BC DOM were portioned into <1 kDa fractions, whereas the major chromophores were partitioned into >5 kDa fractions. The >5 and 1~5 kDa fractions in BC DOM produced at 400, and 500 ℃ were generally contained higher MW and aromaticity than those at 300 ℃. The >5 kDa fractions within rice straw-derived BC DOM contained more aromatic structures than pig manure-derived ones, while the <1 kDa fractions within latter ones contained more aromatic ones. It was worth noting that MW fractions in rice straw and pig manure-derived BC DOM almost exhibited similar EEM spectra characteristics, indicating that BC DOM were complex organic compounds with a chemical continuum. Additionally, EEM spectra showed that fulvic-like and short-wavelength tryptophan-like fluorophores were predominant in MW fractions within rice straw- and pig manure- derived BC DOM, respectively. With increasing MW fractions, fluorescence index (FI) and autochthonous index (BIX) of DOM decreased, while humification index (HIX) increased, implying that high MW fractions enriched organic matters with high aromatic and humification degree. The study enhanced our understanding of the molecular composition and structures of BC DOM, which could also provide beneficial references for accurately evaluating environmental behaviors of BC DOM.