Astronomical focal-plane infrared detectors are generally installed in cryogenic Dewars. To measure the detector's dark current, all infrared backgrounds must be shielded. Hence, excluding radiations from the Dewar windows without modifying the Dewar structure is challenging. Herein, we introduce a revised method to measure the dark current by modifying the temperature of the Dewar window without disassembling and assembling the Dewar. Test results of a 512×640 near-infrared focal-plane detector show that the revised method can obtain dark currents that are 10 e^-/s lower than those afforded by conventional methods without excluding the infrared background from the Dewar windows. The associated revised test procedure can be used to promptly evaluate the dark-current performance of the detector.