AbstractThe bottom‐up preparation of two‐dimensional material micro‐nano structures at scale facilitates the realisation of integrated applications in optoelectronic devices. Fibrous Phosphorus (FP), an allotrope of black phosphorus (BP), is one of the most promising candidate materials in the field of optoelectronics with its unique crystal structure and properties.^[1] However, to date, there are no bottom‐up micro‐nano structure preparation methods for crystalline phosphorus allotropes.^{[1c, 2]} Herein, we present the bottom‐up preparation of fibrous phosphorus micropillar (FP‐MP) arrays via a low‐pressure gas‐phase transport (LP‐CVT) method that controls the directional phase transition from amorphous red phosphorus (ARP) to FP. In addition, self‐powered photodetectors (PD) of FP‐MP arrays with pyro‐phototronic effects achieved detection beyond the band gap limit. Our results provide a new approach for bottom‐up preparation of other crystalline allotropes of phosphorus.