Peak-to-average power ratio (PAPR) reduction is a challenge for intensity modulation/direct detection (IM/DD) optical orthogonal frequency division multiplexing (OFDM) systems. Selective mapping (SLM) is a classical algorithm that can effectively reduce PAPR with high complexity. We propose an improved SLM scheme which employs discrete multitone modulation (DMT) to obtain several candidate signals through an inverse fast Fourier transform (IFFT) and cyclic shift addition operation, thereby reducing the computational complexity significantly. Both simulation results and experiments show that the proposed scheme achieves better PAPR reduction performance with low computational complexity and improves the receiver sensitivity.