Narrowband photodetectors (PDs) with remarkable comprehensive performance are extensively in demand in critical applications such as spectrometers, light detection and ranging (LiDAR), optical communication, and surveillance. Despite the fact that silicon-based narrowband PDs currently predominate the market, their inherent rigidity, fixed band limitations, and bulky sizes significantly impede their applications in emerging fields, particularly in flexible and wearable optoelectronics. In this work, a facile and universal solution-processed strategy is presented to fabricate narrowband organic PDs (OPDs) that can detect arbitrary narrow spectra, which is realized by introducing an independent organic optical adjusting layer (OAL) comprising the same donor and acceptor within the photoactive film of OPD. With this strategy, we successfully fabricate a narrowband OPD centered at 750 nm, with a full width at half-maximum (FWHM) of 40 nm, spectral responsivity ratio of 3667, and -3 dB cutoff frequency of 927 kHz, as recorded in self-powered narrowband OPDs. Notably, this novel OAL strategy shows outstanding universality across all OPDs for detecting arbitrary narrow spectra in the range of 600-1000 nm with tunable FWHMs (minimum narrowing step of 2 nm), high responsivity and detectivity, and fast response speed. Benefiting from the inherent flexibility of organic semiconductors, we fabricate a flexible and bendable narrowband near-infrared OPD linear array for omnidirectional LiDAR, realizing a 360 degrees horizontal scan without mechanical rotation.