It is indispensable to make use of guard interval to overcome intersymbol interference (ISI) caused by chromatic dispersion for optical orthogonal frequency division multiplexing (OOFMD) . However, the channel delay is not constant in different transmission distances and conditions, fixed guard interval decreases the power and spectrum availabity considerably. A novel adaptive cyclic prefix (ACP) OFDM concept is proposed. The lower limit value of CP, which can be determined by differential mode delay （DMD） and transmission distance, can eliminate influence of ISI. Simulation results demonstrate that the system can transmit a data rate 10 Gb/s about 1 km with reasonable CP without sacrificing the BER performance compared with the conventional non-adaptive CP.