In this paper, the field-free alignment of molecule ClCN is investigated with a terahertz few-cycle pulse (THz FCP) based on the time-dependent density matrix theory. It is shown that a high degree of molecular alignment can be obtained by changing the matching number of the THz FCP in both adiabatic and non-adiabatic regimes. The matching number can affect both the maximum value of the alignment and the time at which it is achieved. It is also found that a higher degree of alignment by the THz FCP can be achieved at lower intensity and there exists the optimization threshold of molecular alignment with the increasing of the field amplitude. Also found is the frequency sensitive region in which the degree of maximum alignment can greatly be enhanced through the modulation of the center frequencies of different THz FCPs. The investigation demonstrates that comparing with a THz single-cycle pulse, a better result of the field-free alignment can be created by a THz FCP at the constant rotational temperature of molecule.