Upon the analysis of the existing generation methods for the optical spectrally efficient frequency-division multiplexing (O-SEFDM) system, a generation method based on the modified universal inverse fractional Fourier transform (uIFrFT) is proposed. On the basis of uIFrFT with P symmetry, Hermitian symmetry can be applied to generate real-valued signals. Therefore, the up-conversion in the intensity modulation/direct detection (IM/DD) O-SEFDM system is avoided, which simplifies the system. Meanwhile, a suitable spherical part detector (SPD) is designed in this scheme. The simulations demonstrate that the generation method based on uIFrFT outperforms the existing generation methods in flexibility and performance for the IM/DD O-SEFDM system. Furthermore, SPD has improved the bit error rate (BER) performance and reduced complexity by about 51% compared to the conventional fixed spherical detector (FSD) in fourth-order quadrature amplitude modulation (4QAM) and 16QAM transmission systems.