The coupled-mode equations in photonics crystal fiber with triangular structure triple-core (TTC-PCF) are achieved based on the coupled-mode theory, thereby the directional coupling between the cores in this structure is numerically studied in detail. The influences of the fiber structure and the incident wavelength on the coupling coefficient, and the effect of the amplitude ratio of the inject beams at the input end on the transfer of energy between the cores are analyzed. From the results, it can follow that continuous modulation of the coupling intensity between the cores can be achieved by changing the amplitude ratio of the inject beams. The coupled-mode theory and the beam propagation method present the results well consistent with each other. The unique coupling transmission performance of the TTC-PCF shows its possible applications in the design and the preparation of coupling strength continuously tunable fiber directional coupler and large mode field fiber lasers.