Unification of the frequency shift and pulse compression of an ultrashort laser pulse propagating in a background plasma wave is realized by employing the concept of phase modulation of the laser pulse by the plasma wave. The equation for the evolution of the pulse width is obtained, which is analogues to the "energy conservation equation" for a classical particle moving in a potential well. The threshold amplitude of plasma wave for the pulse compression is estimated. Numerical calculation is performed to confirm these results. The possibitity of laser pulse compression by plasma wave in practice is suggested finally.