Black phosphorus (BP) has attracted great attentions in the last few years, but their applications in optoelectronics have been strongly hindered by the poor environmental stability, especially under light irradiation. Herein, the fabrication of surface passivated few‐layer BP assisted by polymer ionic liquids (PILs) is reported. The PIL‐modified 2D BP exhibits excellent stability under ambient conditions, with negligible deterioration in 100 d. Furthermore, the PILs attached on the BP surface provide reliable and flexible electrical contact between the few‐layer BP and other device components. As a proof of concept, PIL‐modified BP nanosheets are successfully applied in flexible photodetectors, which show high flexibility, good detectivity with no obvious performance deterioration in 120 h. This work demonstrates that the PIL modification can endow BP nanosheets with excellent environmental stability and good conductivity, so that may significantly expand the applications of BP in flexible optoelectronics. An efficient strategy to fabricate polymer ionic liquid (PIL)‐passivated 2D BP is developed. The PIL‐modified 2D BP shows negligible structural deterioration in 100 d under ambient condition. Photodetectors based on PIL‐modified 2D BP exhibit good performance and excellent stability.