In this paper we investigate the phase coherence of hyper-Raman lines in strongly driven two-level systems, and find that when the coherent superposition of excited and ground states is preparated, the high-order harmonics will disappear and the remainder of hyper-Raman lines in the emitted specturm are highly correlated. If the corresponding amplitude and phase of the hyper-Raman lines are selected, a train of subfemtosecond pulses will be generated.