Dissolved organic matter (DOM) exists widely in water environmental media, and its photodegradation process significantly affects the chemical species and ecological risk of pollutants in waters. This study took the typical terrestrial natural organic matter (NOM) and algal-derived organic matter (AlgOM) as the research objects, fractionating the initial DOM sample (Bulk, <0.45 μm) into high molecular weight (HMW, 1 kDa-0.45 μm) and low molecular weight (LMW, <1 kDa) fractions via tangential ultrafiltration technique, to analyze the difference of photodegradation behavior of organic components with different molecular weights in the presence of colloidal TiO₂ particles. The characterization results showed that organic matter of DOM samples was relatively more distributed in LMW components (55.3%-57.8%), but HMW was more aromatic than LMW components. Three-dimensional fluorescence spectrum showed that AlgOM was rich in protein-like and humus-like substances, while NOM only contained humus-like substances. Further analysis revealed that protein-like substances were mainly distributed in HMW components, and humus-like substances were mainly distributed in LMW components. The presence of colloidal particles promoted the photodegradation of organic matter and aromatic substances in NOM by catalytic effect, while inhibited the photodegradation of organic matter and aromatic substances in AlgOM by adsorption effect. Colloidal particles did not change the photodegradation efficiency order of organic matter in DOM with different molecular weights, all of which were HMW>Bulk>LMW, indicating that HMW organic matters were easier to be photodegraded. However, without colloidal particles, the photodegradation efficiency of AlgOM and NOM aromatics was LMW>Bulk>HMW, and the presence of colloidal particles changed the photodegradation order of NOM aromatics with different molecular weights. In addition, terrestrial humus was more easily degraded than algal humus in the absence of colloidal particles, and the presence of colloidal particles promoted the photodegradation of both algal and terrestrial humus. Compared with HMW humus, LMW humus had higher photosensitivity and preferential degradation characteristics.