It is expected that ongoing and future space-borne planet survey missions including Transiting Exoplanet Survey Satellite (TESS), PLATO and Earth 2.0 will detect thousands of small to medium-sized planets via the transit technique, including over a hundred habitable terrestrial rocky planets. To conduct a detailed study of these terrestrial planets, particularly the cool ones with wide orbits, the exoplanet community has proposed various follow-up missions. The currently proposed European Space Agency mission Ariel is the first step for this purpose, and it is capable of characterization of planets down to warm super-Earths mainly using transmission spectroscopy. The NASA Large Ultraviolet/Optical/Infrared Surveyor mission proposed in the Astro2020 Decadal Survey white paper will endeavor to further identify habitable rocky planets, and is expected to launch around 2045. In the meanwhile, China is funding a concept study of a 6 m class space telescope named Tianlin (a UV/Opt/NIR large aperture space telescope) that aims to start its operation within the next 10–15 yr and last for 5+ yr. Tianlin will be primarily aimed at the discovery and characterization of rocky planets in the habitable zones around nearby stars and to search for potential biosignatures mainly using the direct imaging method. Transmission and emission spectroscopy at moderate to high resolution will be carried out as well on a population of exoplanets to strengthen the understanding of the formation and evolution of exoplanets. It will also be utilized to perform in-depth studies of the cosmic web and early galaxies, and constrain the nature of dark matter and dark energy. We describe briefly the primary scientific motivations and main technical considerations based on our preliminary simulation results. We find that a monolithic off-axis space telescope with primary mirror diameter larger than 6 m equipped with a high contrast coronagraph can identify water in the atmosphere of a habitable-zone Earth-like planet around a Sun-like star. More simulations for the detectability of other key biosignatures including O3, O2, CH4 and chlorophyll are coming.