Based on phase-transmission theory, a transmissive terahertz metalens whose wavefront is shaped by cross-shaped elliptical cylinders is designed. The lens uses a cyclo-olefin-polymer substrate and silicon scatterers, has an 11 mm radius, operates at 0.75 mm wavelength, delivers a 0.69 numerical aperture and 51.4% focusing efficiency. Within ambient refractive indices of 1~1.6 and wavelengths of 0.7~0.8 mm, the focal length scales directly with index and inversely with wavelength, enabling continuous zoom by simply tuning the surroundings or the incident frequency.