The demand for satellite communication (SATCOM) has been increasing to provide communication environments for terminals in areas with no terrestrial network coverage and to provide communication means with Internet of Things (IoT) equipment. Accordingly, a few schemes are being studied to efficiently allocate power resources with multi-beam directivity control through digital beam forming (DBF). Moreover, power resource allocation with simultaneous control of the transmit power and multi-beam directivity can be expected to increase the flexibility according to traffic demand. However, there is a trade-off between the transmit power and beam directivity under some physical constraints, and currently there is no mathematical model to solve this problem. The contributions of this letter are to construct a new mathematical model necessary for solving the trade-off, and to introduce a novel power resource allocation with fusion control of the transmit power and multi-beam directivity according to traffic demand with the objective of improving the allocation efficiency of communication resources. We carried out a numerical analysis to verify the effectiveness of our proposal.