An NMR analysis of ^31P nuclei in metallic iron phosphide FeP has been performed both in a zero field and by sweeping the external magnetic field at several fixed frequencies with the aim of revealing and investigating the transformation of the magnetic helical structure of FeP. The NMR lineshape gradually changes with an increase in the field strength from trapezoidal (in weak fields) to pronounced asymmetric with two peaks (in strong fields). The former shape is typical of NMR spectra of powder samples under an applied magnetic field, while the latter is characteristic of NMR spectra of nonmagnetic atoms in single-crystalline helimagnetics. The observed transformation of the FeP NMR spectrum evidences in favor of the spin-reorientation transition of a spin-flop type in FeP at strong external fields.