Based on the simplest two-fluid dynamical model neglecting the phonon-roton and roton-roton interactions but taking only the terms essential to superfluidity into account, the dynamical structure factors S( Q, ω) in the region Q≤0.8A -1 are obtained and calculated numerically from 1.4K to T λ. The obtained S( Q, ω) have spectra of phonon type functions with the resonance energy ɛ Q (ω) and the width Γ Q(ω) are given with the expressions which are strongly dependent on the frequency. In this model S( Q, ω) can be written as the separated form, S(Q, ω)=S L(Q, ω)−S L(Q, -ω) , where S L( Q, ω) is Lorentzian type function with the resonance energy ɛ Q(ω)=√ɛ Q(ω) 2−Γ Q(ω) 2 and the width Γ Q (ω). Though the ɛ Q (ω) and Γ Q (ω) are ω-dependent in this model, it is shown that S L ( Q, ω) has the same spectra as S( Q, ω), and also that the peak frequencies of S( Q, ω) (and S L ( Q, ω)) are nearly T-independent. Further considering the sum rules we discuss the validity of the simple Lorentzian type of S L ( Q, ω) in which the resonance energy and the width are ω-independent.