If a metalens integrates the circular polarization (CP) conversion function, the focusing lens together with circular-polarizing lens (CPL) in traditional cameras may be replaced by a metalens. However, in terahertz (THz) band, the reported metalenses still do not obtain the perfect and strict single-handed CP, because they were constructed via Pancharatnam-Berry phase so that CP conversion contained both left-handed CP (LCP) and right-handed CP (RCP) components. In this paper, a silicon based THz metalens is constructed using dynamic phase to obtain single-handed CP conversion. Also, we can rotate the whole metalens at a certain angle to control the conversion of multi-polarization states, which can simply manipulate the focusing for incident linear polarization (LP) THz wave in three polarization conversion states, including LP without conversion, LCP and RCP. Moreover, the polarization conversion behavior is reversible, that is, the THz metalens can convert not only the LP into arbitrary single-handed CP, but also the LCP and RCP into two perpendicular LP, respectively. The metalens is expected to be used in advanced THz camera, as a great candidate for traditional CPL and focusing lens group, and also shows potential application in polarization imaging with discriminating LCP and RCP.