Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a promising candidate for photodetector (PD) applications thanks to its excellent optoelectronic properties. In this work, a green solution- processed spin coating and selenization-processed thermodynamic or kinetic growth of high-quality narrow bandgap kesterite CZTSSe thin film is developed. A self-powered CZTSSe/CdS thin-film PD is then successfully fabricated. Under optimization of light absorber and heterojunction interface, especially tailoring the defect and carrier kinetics, it can achieve broadband response from 300 to 1300 nm, accompanied with a high responsivity of 1.37 A/W, specific detectivity (D*) up to 4.0 × 1014 Jones under 5 nW/cm2, a linear dynamic range (LDR) of 126 dB, and a maximum Ilight/Idark ratio of 1.3 × 108 within the LDR, and ultrafast response speed (rise/decay time of 16 ns/85 ns), representing the leading-level performance to date, which is superior to those of commercial and well-researched photodiodes. Additionally, an imaging system with a 905 nm laser is built for weak light response evaluation, and can respond to 718 pW weak light and infrared imaging at a wavelength as low as 5 nW/cm2. It has also been employed for photoplethysmography detection of pulsating signals at both the finger and wrist, presenting obvious arterial blood volume changes, demonstrating great application potential in broadband and weak light photodetection scenarios.