Four new secondary metabolites, chaetosemins G-J (1-4), along with 11 known ones (5-15) were isolated from the culture of C. seminudum C208 and Chaetomium sp. C521. Their structures were determined by extensive NMR spectroscopic analyses. These metabolites were evaluated in vitro for antifungal, antioxidant, toxicity, and α-glucosidase inhibitory activities. Chaetosemin J (4) and monaschromone (5) significantly inhibited the growth of four plant pathogenic fungi Botrytis cinerea, Alternaria solani, Magnaporthe oryzae, and Gibberella saubinettii with the minimum inhibitory concentrations (MIC) values ranging from 6.25 to 25.0 μM. Moreover, both epicoccone B (11) and flavipin (14) exhibited the DPPH free radical scavenging ability with IC50 values of 10.8 and 7.2 μM, respectively, and had more potent α-glucosidase inhibition than the drug acarbose with IC50 values of 27.3 and 33.8 μM, respectively. Monaschromone (5) might act as the lead compound of pesticide.