The demand for an ultrabroad optical material with a bandgap tunable from zero to at least 1–2 eV has been one of the driving forces for exploring new 2D materials since the emergence of graphene, transition metal dichalcogenides, and black phosphorus. As an ultra-broadband 2D material with energy bandgap ranging from 0 to 1.2 eV, layered PtSe2 shows much better air stability than its analogue, black phosphorous. In this work, high quality of centimeter scale PtSe2 films with controllable thicknesses were prepared through thermally assisted conversion method. The linear and nonlinear optical performance and ultrafast dynamics of layered PtSe2, and signatures of the transition from semiconductor to semimetal have been systematically studied experimentally and theoretically. Combining with rate equations, first-principles calculation, and electrical measurements, a comprehensive understanding about the evolution of nonlinear absorption and carrier dynamics with increasing layer thickness is provided, indicating its promising potential in nanophotonic devices such as infrared detectors, optical switches, and saturable absorbers.