Lenses with desired depth of focus have crucial applications in imaging systems. However, there is little theoretical guidance to extend the depth of focus beyond numerical optimization. The on-demand construction of the Jones matrix using the composite metasurface brings a powerful tool for polarization-multiplexed functionalities. Here, based on polarization-multiplexed focusing in four linear polarization channels, we propose a straightforward method to extend the depth of focus based on the coherent superposition of each linear polarization channel. The metalens shows long and uniform needle beam focusing with a depth of focus of 46λ in circularly polarized excitation in the experiment, which offers a promising tool to tailor the terahertz focal spot for imaging applications.