The lepton-number-violating decays of singly-charged Higgs bosons H~± are investigated in the minimal type-(Ⅰ＋ Ⅱ ) seesaw model with one SU(2)_L Higgs triplet △ and one heavy Majorana neutrino N_1 at the TeV scale. We find that the branching ratios B(H~+ → 1_α~+(γ)) (for α = e, μ,τ) depend not only on the mass and mixing parameters of three light neutrinos γ_I(for I= 1,2,3) but also on those of N_1. Assuming that the mass of N_1 lies in the range of 200 GeV to 1 TeV, we figure out the generous interference bands for the contributions of γ_I and N_1 to B(H~+→1_α~+(γ)). We illustrate some salient features of such interference effects by considering three typical mass patterns of γ_I, and show that the relevant Majorana CP-violating phases can affect the magnitudes of B(H~+→1_α~+(γ)) in this parameter region.