BIB (Blocked Impurity Band) detectors operating at deep cryogenic temperatures have significant application potential in fields such as infrared astronomy and space observation. However, studies on their temperature-dependent performance mechanisms remain relatively limited. In this work, a planar p-i-n long-wavelength infrared BIB detector based on high-purity germanium was fabricated using a near-surface treatment technique. The device exhibits excellent electrical and photoresponse performance at relatively elevated temperatures, achieving an increase of approximately 10?K in operating temperature compared to conventional BIB detectors. At 3.3?K, the reverse-bias dark current is as low as 15?pA. With increasing temperature, the blackbody detectivity shows a decreasing trend, but remains nearly constant below 15?K, reaching up to 3.5×1012?cm.Hz1?2.W?1. A current model incorporating photoexcitation, thermal excitation, and impact ionization processes was introduced to simulate the device behavior. The simulation results agree well with the experimental data and reveal that the primary degradation mechanism is the significant shrinkage of the depletion region at elevated temperatures, which reduces carrier collection efficiency. This study provides both theoretical support and experimental evidence for the structural design and performance optimization of BIB detectors for low-temperature infrared applications.