Flexible photodetectors have attracted lots of interest due to the advantages of wearability, portability, and implantability, thus adapting to various substrates or skin surfaces for wearable applications. Organic semiconductor materials are a promising candidate as photoactive layers for their excellent photoelectric properties, remarkable mechanical flexibility, and outstanding solution processability. Currently, flexible organic photodiodes (OPDs) have made important progress and have been comparable to their rigid counterparts in terms of partial device performance. In this review, the developments of flexible OPDs from the aspect of detectability and mechanical performance are summarized, including flexible substrates, flexible electrodes, and active layer films. Furthermore, common applications like health monitoring, optical communication, and image sensing are further exhibited. In the end, the challenges and development opportunities of the flexible OPDs are prospected.