We report on observation of coherent longitudinal acoustic phonons in Fe/AlGaAs (001) heterostructure by time-resolved second-harmonic generation (TRSHG). The phonon induces not only a fast and highly damped oscillatory signal but also a long-lived one that exhibits an oscillating period identical to that obtained from the reflectivity measurement. The long-lived oscillatory second-harmonic generation is generated by an electric field modulated at 42.5 GHz because of the interference of the incident beam and the reflected light by the propagating phonon. TRSHG lets us probe the internal field modulated by phonons and differentiate their impacts at the interface and in the bulk.