All inorganic perovskite CsPbX3 with excellent optical properties and a tunable bandgap is a potential candidate for optoelectronic applications, and the amplified spontaneous emission (ASE) is normally reported in low-dimensional structures where the quantum confinement enhances ASE. Herein, we not only demonstrate the ASE in millimeter size CsPbClxBr3-x crystal with a high defect concentration, but also tune the emission wavelength from the green band to blue band through the ion exchange of Br with Cl. The ASE centered at ∼456nm is probed at 50 K with a threshold of 106μJ/cm2. Furthermore, a metal-semiconductor-metal (MSM) structure CsPbClxBr3-x photodetector is fabricated and shows a distinct response to lights from UV to the blue band; the response spectrum range is quite different from the narrow band (∼30nm) response of the CsPbBr3 photodetector induced by a charge collection narrowing (CCN) mechanism. The CsPbClxBr3-x photodetector also exhibits fast response speeds with a rise time of 96 μs and a decay time of 34 μs, indicating the defects have limited influence on the transportation speed of the photo-generated carriers.