The form E(t)=f(t) sin (ωt+) is often adopted to describe a laser pulse in the simulation of the physical process of laser interaction with matters. In the present paper, the validity of this expression and found it skeptical to model a pulse with the width less than a single optical cycle. The form E (t) = f (t) sin (ωt + ) is often adopted to describe a laser pulse in the simulation of the physical process of laser interaction with matters. In the present paper, the validity of this expression and found it skeptical to model a pulse with the width less than a single optical cycle.