The most serious citrus postharvest diseases are citrus green mould caused by Penicillium digitatum and citrus blue mould caused by Penicillium italicum. Marine microorganisms have become a new measure in the control of these diseases. In this study, we isolated 56 marine actinomycetes, and strain AM-4 isolated from Drupa granulata exhibited the strongest antagonistic activity against P. digitatum. Molecular, morphological, cultural and physio-biochemical analysis identified strain AM-4 as Streptomyces chumphonensis. Strain AM-4 also exhibited antagonistic activity against 11 other phytopathogens, with an inhibition zone width ranging from 0.65 ± 0.04 to 2.35 ± 0.02 cm. The mycelial growth inhibition rates of the fermentation liquid of strain AM-4 against P. digitatum and P. italicum were 66.23% and 61.42%, respectively, and the conidial germination inhibition rates were 65.77% and 62.13%, respectively. The relative control efficacy of the gradient-diluted fermentation extracts of strain AM-4 against citrus green mould caused by P. digitatum ranged from 77.95% to 92.91% at 5 days post inoculation and 56.30% to 87.22% 10 days after inoculation. The relative control efficacy of the gradient diluted fermentation extracts of strain AM-4 against citrus blue mould caused by P. italicum ranged from 61.32% to 84.91% 5 days after inoculation and from 51.34% to 80.92% 10 days after inoculation. The relative control efficacy of the fermentation extracts of strain AM-4 was even better than prochloraz against citrus green mould caused by prochloraz-resistant P. digitatum. Therefore, as a biological control strain, AM-4 has a good prospect for development and utilization potential.